Being around that kind of age, I know a few lads who do it. Some, like me, are still playing five-a-side as well as walking football. You need to be over 50 for most of the games I've heard of, and I'm told the standard is pretty reasonable, and it can be fairly competitive without being daft.
Certainly the chaps I know who play were decent players in their own right, and from a level beyond that at which I've played (e.g Cov Sporting, Folly Lane, those kind of teams). They play two-touch, so there's still an emphasis on controlling the ball and a bit of skill involved. On that basis alone I'd struggle, but in every other sense I've been preparing for walking football for my whole career.