Today I ride a cyclocross bike to commute. I am wanting to retire it from that task so I can put it back to work learning the game of CX, and getting back into racing (I race backwards, trying to avoid losing - DFL - rather than winning) CX. I have an opportunity for a steal of a deal on a 26" MTB. I know the rage today is a 29er, but this one is such a good deal I may not be able to make myself pass it up.

I have an 8-mile commute. The return has three substantial hills. I usually get to work in about 30 minutes, home in 40 or so.

Is the MTB going to significantly lengthen my commute time?

Will I be able to mount my panniers so that I don't have that danged heel strike on the bags?

Anything else worth considering that I maybe haven't?

The bike is a 2011 Giant Revel, mechanical disk brakes, $200.

Or should I save more for something else?

I appreciate your advice (and I am a year-round commuter, using my road bike in summer weather).