I did my first 100 mile sportive 50 days after getting my road bike, I had barely ridden my mtb up to that point. So pretty much was starting from scratch. I went out 2-3 times a week (this was between august and October so better weather and light evenings) upping my mileage steadily until my last training run of 70 miles.
I would just say take it easy as I probably rushed it and it used to hurt a lot between rides - stiff legs, back, neck and shoulders! But gradually as you get fitter and better at it, those things subside.
Funnily enough for me it takes at least probably 7-8 miles before I start to feel comfortable and the numb bum goes away.
Since the 100 miles in early October I've done bugger all riding except for a few turbo runs, so hoping to pick it up again in the spring - fair weather biker!