Billy,
I don't think you'll have any problems packing around a 25 lb. Backpack. Mine was considerably heavier on my last trip (probably over 50 lbs.) and I managed just fine (and I'm in my late '50s). I'm a firm believer in using a properly fitted Backpack for my torso size, so the waist belt takes most of the weight.
Keep in mind that for the most part you'll only be using the main Pack from the train to your Hostel or Hotel, so you won't be carrying it for long periods of time.
I'm assuming from your Post that you're planning to check the main Pack during air travel due to the weight. If your Pack doesn't have a "stowable harness", it's a good idea to secure the straps in some way so they aren't damaged by airport conveyor systems.
I'm thinking of trying the "one bag" travel method on future trips and will soon be ordering a similar type of bag that Frank II mentioned. I've spent numerous hours over the last few nights revising my somewhat extensive packing list to see if I can reduce it enough to fit everything into one bag. In reality I'll probably end up with two bags, as I always travel with Camera gear.
You mentioned having foot problems when you did a "test run" with your Pack. Be sure that you have comfortable, well broken in shoes! I've had foot problems when travelling, so I know how serious these can be.
Happy travels!