I certainly wasn't advised not to cycle during salvage RT, and in fact I cycled to the hospital for the first threequarters of the treatment. For the last week though (it was a 20 fraction/4 week treatment) I had a big drop in energy levels and used the car instead.
It took about a month after the end of treatment before I felt able to resume normal exercise levels (though to be fair my cycling level, using a hybrid, is rather below yours).