is this a completely stoopid idea?

Since the Bike Events brochure came in the post, I've been thinking
about doing the Manchester 100 ride in September.

I'm a very overweight 30-something who mainly uses her bike for
pootling to the shops and such. The furthest I usually ride is about
5 miles. Oh and my bike is crap (tho there is a possibility I might
be getting a shiny new one in July).

How realistic is it that I could be able to cycle 62 miles in one day
by September? Obviously I'd need to train/practice, can any suggest
how much and how often?

ta

Audrey