Right im 14st of unfit flesh and for some crazy reason I agreed todo the Cardiff 10K run for Marie Curie. Ive never been one for any sort of regular exercise and I know if I attempt this in my current state of fitness, I will die

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Anyone got any good starting pointers? I do a fair bit of street riding in the summer months, but the bike aint been touched since last year. I need some sort of flexible training as ive got quite a demanding job and a nipper to look after outside work.
Cheers