Advice on how and what to see in Croatia and Slovenia.

I am not acquainted with Croatia, but Slovenia has a useful rail and bus network, including connections to Venice and - one of my favourite cities - Trieste. You'll love Lake Bled, Ljubljana and small provincial towns like Maribor.
Almost everybody in Slovenia seems to speak English, including the people at ticket windows, and you'll find it a most pleasant easy-going country.

You're smart to want to see something besides the coast. The inland towns usually get far fewer tourists (Ljubljana being a major exception, but it's worth being part of the tourist scrum).

Croatia has some train service (Google Croatia Rail Map) as far south as Split. The line doesn't hug the coast. Buses provide more complete coverage and are comfortable. English is widely spoken, at least by those under 40, so the country is easily managed by solo travelers. Just check outbound bus schedules as soon as you arrive in each town to be sure you know the current situation. You'll be traveling during a period when there could be some schedule changes, and I don't trust online bus schedules to be totally accurate.

Zagreb, well-connected to Ljubljana (Slovenia) by rail and bus, is a city I really enjoyed. It has a large, bi-level historic district and lots of good museums (including a superior though small museum of naïve art and the quirky but fun Museum of Broken Relationships. There's an active café culture that makes it lively, but the vibe is different from Ljubljana's, perhaps because Zagreb's historic district doesn't have a river running through it. If you want to start your trip in Croatia or Slovenia, definitely check out flight options into Zagreb, which may be less expensive than flying into Ljubljana.

There's shuttle service (I think by GoOpti) from Ljubljana to Venice, or you can cobble together a rail, bus or (probably) combo routing.

The Plitvice Lakes National Park is magnificent, but it is critical to spend the night before your visit in or near the park. It gets slammed by day-trippers to the point that the experience is, frankly, far inferior to what it should be. There's bus service to the park from both Zagreb and Split. If you time your arrival for mid-afternoon, you might enjoy a few pleasant hours in the park that day, then finish up early the next morning. I made the mistake in 2015 of not doing what I am recommending, so I can offer no specifics on where you might stay. Booking.com does have a lot of listings in the area, and I think you might find a small place outside the park (and thus less expensive) that offers shuttle transportation; obviously you'd want to check into that carefully. If you can manipulate your schedule so you visit the park mid-week, that might also be helpful. It was my impression that a lot of the park visitors were Croatians.

The Istrian Peninsula is lovely. The coastal towns of Piran (Slovenia), Rovinj, Porec and the tiny Vrsar are worth visiting. Rovinj and Porec have totally different appearances. The one-day bus tour I took through Inland Istria was a very efficient way to see an area that is not well-served by public transportation; it was worth being tied to the tour schedule to be able to get to so many places in one day. Two key stops were the medieval towns of Motovun and Groznjan. That bus tour ran only on Wednesdays in 2015. This seems to be the tour I took; it's quite reasonably priced at 60 euros.

I also liked Zadar. I haven't been to Split or Dubrovnik recently.

How much time will you have for your pre-tour explorations?

Dejan has been very generous in describing what there is to see in Slovenia. It's worth looking for earlier posts in the Slovenia forum. I haven't seen much of that country due to an untimely illness that pinned me down in the lovely Ljubljana for nearly a week on my last visit. The Skocjan (more natural) and Postojna (more touristy, with colored lights) Caves are worthwhile, but I'd just pick one. The Vintgar Gorge is highly developed for tourism but makes for a nice couple of hours or so out in nature. All reachable by public bus but Skocjan may also require walking 1+ miles.

I have not made my plane reservations...so am flexible on length beforehand. I did read that Croatia is very mountainous, and thus strenuous...I am just recovering hiking strength after surgery, so want to be somewhat cautious. I can walk all day, however, with no problem. Thank you to you all who have taken your time to advise me..what kindness!

Both countries are mountainous, Slovenia is even partially in the Alps. However, just because the terrain is hilly you won't necessarily have to do a lot of hiking. There are hikes, but most tourist areas don't require hiking or the walks that are required are flat. Furthermore, most towns and cities in Slovenia and Croatia are quite flat, too.

It's hard to give concrete suggestions without knowing a bit more about your trip, but it wouldn't be too difficult to do a trip around both countries by public transport while avoiding inclines. The others have provided you with some excellent suggestions and I would just add that you shouldn't be concerned about being a solo female traveller. Both countries are very safe, just take the usual precautions and you will be fine.

We discussed the Vintgar Gorge here not too long ago, and we decided that it was a relatively flat walk. The walk around Lake Bled is, too. There's a bit of an incline down to the entrance to the Skocjan Caves, and there are steps inside (with good handrails, as I recall). Postojna was so long ago that I don't remember conditions there.

I've seen somewhat more of Croatia, and what might be a concern is the previously-mentioned crowding on the walkways at Plitvice Lakes National Park. There are very, very few railings. You're never far above the water, but who wants to fall even one or two feet into a shallow lake with a probably uneven surface? Most of the paths that are not over water are on uneven ground, and many are not paved (wet leaves possible). Do everything you can to avoid crowded times at the park. The crowding was a more-than-average concern to me on my visit because I had suffered a severe sprain a few weeks earlier and didn't trust my ankle. There were people there with dogs--on leashes and very well-behaved, but still.