Oh, I should point out that riding a mile at 22 mph is not exactly easy, that's about 300 watts (if in the drops) for 3-4 minutes, I daresay many 50+ riders can't do that (and especially not after a session of radiation, fer chrissakes). So it's not like you're starting from square one. But anyway, since your cardio seems capable of more, I'd say build up those legs. 17 mph is around 170 watts, so it makes sense to me that legs that are used to working at that pace would quail when suddenly told to work at 2X that pace, they're not used to it - but you can make them get used to it.