Barcelona to Madrid and Toledo back to Barcelona

From Barcelona to Madrid the Regional takes 9 hours the AVE takes 3 hours. You can by the AVE the day of it just costs more.
You can buy Madrid to Toledo anytime. It is a short 30 minute ride

1.) There are no slow trains between BCN and MAD (there haven't been for years). Now all the trains are fast speed trains (AVE or similar). There are over 35 of them on any given day from early in the morning until late in the evening -less on weekends, of course! The journey is around 2:30h-3h depending on the number of intermediate stops. There is a handful of them (maybe half a dozen) that take 6h because they go first to Valencia, then you transfer to another train to Madrid.

2.) For info: as per RENFE's jargon, "regional" trains serve only cities within the same region (up to a max of 300km). Barcelona is in Catalonia and Madrid is in Comunidad de Madrid... two separate "regions". Therefore I am afraid there are no "regional" trains between BCN and MAD. For further info: in the Spanish Rail System, trains are grouped in high speed (AVE, Alvia...), MD (medium distance, connecting different provinces within the same region and sometimes with provinces from neighbouring regions), Regional and Regional Express (connecting stations in the same region or Autonomous Community) and, finally, Cercanías (or Rodalies in Catalonia, read commuting lines, which connect destinations within the same province).

3.) While it might be possible to find a ticket for a fast train from Barcelona to Madrid on the same day... I wouldn't risk it as it's not normally the case for this particular line, which is the most heavily used in the Spanish rail system. Nearly 65% of the trips between these two cities are now made by high-speed train (vs airplane) with a total of 3.8 million passengers in 2016.

4.) Booking on the same day on this particular busy line, assuming you find a ticket on the train you need to take, can sometimes -on key dates- cost you a bit of an arm and a leg.

Just sayin'

PS: For sake of clarity: Autonomous Community = US State, Province = County (albeit larger than your average counties) and, albeit it doesn't exist as a political division per se, the word Region can be assimilated to Autonomous Community. See the map of Spain's Autonomous Communities to get a better idea: https://tutora3bprimaria.files.wordpress.com/2016/01/mapa-politico053.jpg

update Thanks Joan for pointing, but being an "exceptional exception" (there's only one among more than 35 trains every day!) I didn't consider worth mentioning moreover noting that it "wastes" a full day for someone visiting since the 10h+ journey it's not overnight, but during the day.

Of course there is a Regional train from Barcelona to Madrid. It is the TransMesetario, and it takes 10 hours! It is a lovely journey on an old train, but really slow. This train costs 41€ the entire way, but you do not need to prebook.
Otherwise, I would take the AVE. You should prebook AVE trains online at www.renfe.com, because you might not find a ticket or it might cost a lot of money. In any case, if you buy a Promo+ fare, you have certain flexibility when cancelling or changing your ticket.
Note for EnricM: trains in Spain:
Regional: sometimes they go out of the Autonomous Community. The main description is: stops at all stations.
Regional Express: Regional train that only stops at key stations. More expensive than a Regional "basic" fare.
MD: train that is served by 449 or 599 series (the newest train for regional services). MD tickets include a surcharge on their fare for using this newer trains.