The Start of a Long Run

Here we are, mid November, i've completed the London to Paris bike ride, i've completed my first triathlon and now I have a Marathon to train for. The other two events were easy in comparison to this new event I have infront of me. 26.2 miles is a long way and having completed the distance twice in the past I know how hard it is going to be. The difference now though is that I am over 10 years older than when I last completed the distance so now it is going to be even harder. Foolishly, because I was lazy, I didn't really train when I completed them before and then I was able to finish in just over 5 hours. Now I have a target, my aim is to complete the course in about 4 hours, 4 and a half hours max. But how am I going to do it? I know i'm meant to do it by lots of training, but i'm not quite sure how i'm going to get the motivation to do it. Here's hoping.