Converting Mountain Bike for Road to complete in 33 mile mini triathalon I have a very good mountain bike and like to ride trails.  I recently signed up for a mini triathalon that requires a 33 mile road ride.  I don't want to buy a new bike, so I was thinking about buying a new set of rims and swapping them out for this race.  Not very experienced with converting, so looking for some help on solutions.  Don't want to spend a ton of cash on this either.
Even if you do not go as far as different rims, consider different tires.  

Slicks (1.25") vs the regular tires (2+ inches) will make a huge difference, and usually can be found for $30 a tire or so in the US or Canada.

Beyond that, remove unnecessary hardware.

Front shocks are probably the worst thing you might have on the bike.  I do not know if it is worth considering a hard fork just for this race or not.