Mountain bikes on road rides

[Trek420]

Yes, it's official, I suck at training. I've spun, crosstrained, put in the base miles but nowhere near what I planned. Once more here comes Bike Against the Odds and I'd like to do better than last year.

It's a hilly ride, my fabulous road bike is geared for hills. I don't know the exact count but my low gear is really low. But would it be better for me to take the MTB on hills that have kicked my happy butt before?

The MTB has slicks, new and much more better wheels and low gears of course. I could probably keep forward motion at 0.25 mph

On the theory that as the Great Eddie said "you're faster going than not going" would a different bike get me through the metric century. I only did the 47 last year.