Is it easy to walk around Frankfurt?

I don't know what cities you've been going to but they must be small ones, the main cities in most European countries (including Frankfurt) you can't walk from one end to the other.

But if you're staying in the centre than you don't necessarily need to go further than walking distance to see the sights and experience the city, but if you wanted to go to the edge of the city or out of the city then yes you would want to catch a train or bus

Frankfurt is a big city and they have a good transport

Like so many cities, it has a center which is not too big, I did walk that even when my condition was not the best.

But there are outlying area with housing and hotels can be rather far out of the city center, if yours is, you will want to use the bus or underground.