Anyway, it seemed like cycling fitness transferred quite nicely as I was able to run the four miles up into town and down to the shore in about the same time as it would have taken on the bus - without getting out of breath either... result!
Unfortunately, the following day I awoke with what can only be described as "beaten with a baseball bat" legs, sides, and even back (!!) - I could hardly negotiate stairs, and even walking felt like a verge-of-death experience! It took until today to feel even half capable again. Having done a little Googling it seems that most beginner training programmes boast they can get you to jog 30 minutes after only 10 weeks build-up... so not the first time then!
So, that's a bit annoying - looks like I'd better resign myself to sitting on the bus with the ill ones.