I suspect the 26 will be fine. When properly set up, a cyclists feet on tippy toe just reach the ground and if you liked it well enough to extend the first ride to several miles that is a very good sign. On the other hand, a dedicated bike shop will have knowledgeable staff who will put you on the right size bike. But even more important, you can go back with any questions about mechanical issues or fit and they will be happy to help.

Muscle soreness is normal for unused muscle. Muscle soreness is also normal if you have been happily riding, say 10 miles and suddenly up the mileage to 15 or 20 miles. The rule of thumb if you are increasing miles is to increase no more than about 10%. Give it a few days rest, then do another 2 miles. Welcome to cycling.

Bern