It seems weird to say it about a player in his mid/late 30s, but when Jimmy does run with ball in hand, there doesn't seem to be much slowing down of the legs
Positioning, reading of play, means he can just jog along and use energy when required.
And probably not doing a lot of hard training between matches.
On a side note, won’t this weekend’s game be his 114th (or 115th) Wasps game? Edges him past the 113 he had at Newcastle. I think that unequivocally qualifies him as a Wasps legend.