Cyclocross - Trek 5000 as a CX?

Okay, so I'm not actually wanting a cyclocross bike, per se -- rather I want to be able to ride my road bike everywhere I go (which involves a lot of dirt roads).

I'm a missionary and development worker in Tanzania, East Africa, and when I moved here I brought both my bikes -- one mountain and one road. The town I live in has one paved road, which is nice for road riding (though a little repetitive); however there are no villages (where my work is) actually on this paved road. So any trip to the bush requires a fair amount of riding on dirt or goat/walking trails.

I've been riding my mountain bike because I know it can handle whatever I put it through, but it's taking me a really long time to reach some of these villages. Four hours in many cases -- and that's just one way. I get especially frustrated when more than half of my ride is on nice pavement, and I'm still rolling on big knobbies, killing myself to get up the hills (and still not able to come down them quickly). I feel like if I could ride my road bike, I could cut my commute almost in half (which might allow me even to go for the day and not be required to spend the night away from the fam).

So... I've got a 2007 Trek 5000 that's only been ridden on the road thus far. I'm currently riding a Bontrager Select wheelset with Bontrager Race Lite 25s. Can I just ride it on the dirt roads as is? Or do I need a sturdier wheel (more spokes)? I'm not sure I can really put any bigger tires on the bike, as the 25s already barely fit between my brake pads when inflated.

And I've read through the forums -- I know a CX bike is probably what I actually need. But I'm in Africa and there's no way to get one here. The less I do to this bike to be able to ride it the better off I'll be. I don't want to get into canti brakes and whatnot. Actually, I'd love to still be able to use the bike as my training bike on the road. [Because neighboring Rwanda has some awesome road riding.] And if I can do it all with the same wheels and tires, I'll be all the happier.

I think mostly I'm concerned with whether or not the bike and wheels will hold up. But I'm certainly open to any advice you guys have got -- especially if you can tell me the practicalities of what PSI to run my tires, etc.

I think it'll take you longer on the roadie due to pinch flats or mechanicals or all the bumps you dont realize your mtb rolls over.

I'd suggest narrower and different tires for the mtb. Something with smoother tread than knobbies in the center and higher pressure limits will roll well on pavement and then knobbed sides will get you traction in the loose. sounds like the trails are fairly tame? No big rock gardens or deep sand? why not go 26x1.75 or so? Maybe you don't need the chunky knobs you think you do?
http://www.amazon.com/Kenda-Bicycle-Gumwall-26-Inch-1-75-Inch/dp/B002DWZD2I
http://www.amazon.com/Kenda-K-Sheild-Bicycle-Blackwall-1-95-Inch/dp/B002DWZE6S/ref=pd_sbs_sg_6

Don't think that switching to a cyclocross bike will halve your commute. If you can only manage 12 mph average, a CX bike won't let you average 24. Might shave off a few minutes, or as the poster mentioned above, increase it due to flats. I agree, try a simple tire swap.

thanks, guys, for the quick replies. i didn't really think riding the road bike would cut the commute in half; only a little hyperbole there. i just know i'd be a lot faster on the pavement portions -- which are often more than half the distance.

thenomad, yeah, the trails are not bad here. it's mostly compact dirt (no big rocks) and not a whole lot of sand. during rainy season there's lots of mud, but there's almost always a line around it. the roads are being used by tanzanians on their bikes (carrying pineapples or firewood, etc) with no problems -- i've thought about getting one of their fixed gear bikes, but they way like a hundred pounds. and i've known i didn't need the chunky knobs i've got; it's just they're all i've got and i haven't had anyone from the states send me any yet in the mail.

but i think i'll have someone send me a few 1.75s (thanks for the links, though the second didn't work -- no worries, i did a search) and give it a go. do you think i could even get away with some 1.50s at a really high PSI? i need to see what these chinese bikes the locals are riding have on them....

so i'll try new tires on the mountain bike, but i'll also try the road bike probably -- as long as no one seems concerned that i'll damage the actual bike doing so. and that doesn't seem to be the case. i just prefer riding with drop bars. also, i do race centuries and ironmans when i occasionally (every two years) return to the states. so i like the idea of riding the road bike a little more often than the other. of course, riding a mountain bike can't be bad training at all for those kinds of events?