I was entered in a marathon later this year. So I decided to get some running training started. I did a few 10 Km runs ( at about 45-50 minutes pace ) and I found that because of the amount of cycling I do, my heart and lungs were fine with it. However the leg muscles really didn’t like it at all, and towards the end of the 10Km runs, it felt like I was running through treacle, and they did need a lot of work the following day, to feel normal again, despite doing all the requisite warm up / warm down routines. After a couple of weeks doing the 10 Km route I had worked out, I pushed it up to 20 Km. The first one was fine, except for the treacle / Haribo legs in the last couple of Kms, when I went to repeat the trick, my left leg went completely dead at about 15 Kms. I limped home, and the following day, I couldn’t walk, I could barely stand. Really severe Shin splints, which were so bad, I thought I may have stress fractured my left tibia, which was bad news, because I had a load of rides to do, which I couldn’t, because I couldn’t move my left leg at all well. It took a week or two to recover, so I’ve binned the idea of running the marathon off, because I can’t risk the potential disruption to my cycling schedule at the moment. That really is a big big difference. The impact / shock your legs are dealing with, really do work the muscles in ways they are never worked when cycling. I used to primarily run ( mixture of road and cross country ) with cycling being the minority activity, so I was surprised at how easily I injured.