Questions!

Okay- so I started mountain biking last year- did some single track and loved it.
Tried winter riding- got my new bike -whoo hoo- and in a VERY freak accident, bruised my ribs and that was it for winter cycling..we had a frightful winter- no soft crunchy snow- oh no...just freezing rain, ice - extreme cold... Cold frozen pavement just feels harder than just plain old pavement, y'know?

But - I'm ready for more cycling this year-
I want to do a lot more.
I'm joining a local group- the first ride's in May -

and I want to do a charity ride in June-
They close off a major highway in Toronto and thousands of cyclists ride it for the Heart and Stroke charity. You'll probably laugh- but I'd like to do 75 k-
the choices are 25, 50 or 75. or more..Any good tips on training?
I'm doing small rides right now- the weather is still really crappy-
but I gotta get my butt in gear so to speak.

I've been doing yoga, planning to do some leg work at the gym-, I'm working on my quads- they're pretty tight. On the average I'm very strong, I have to work on my wind and legs I figure. My knees are shot but cycling doesn't bother them too badly...
however I've been off the bike all winter...

Oh yeah- it'll be on pavement, I'm riding my mountain bike- what about tires? There are Tyranorapter 2.25 knobbys on there now- I've also got a set of 2.1 Jimmy's and a set of IRC Backcountry's.
Or should I invest in a set of slicks for this one event?

Thanks to you all in advance!