if you have (or get) a chain punch, you can shorten the chain your existing sprockets and bypass the rear derailleur and make it into a temporary fixed gear.
first, open the chain, then wrap it around the middle sprocket in front, and try different sprockets in the rear til you find one where the chain links line up when you hold it tight. punch the excess chain off on that link, and put it back together as a fixed speed... you likely have something like a 22/34/42 front, so 34 and one of hte middle gears should hit the sweet spot. me, I'd remove the rear derailleur, and tape the cable back temporarily to try it out, but ou could probably leave it on there. also of course, set the front shifter on MIDDLE! and don't touch them.
now go ride it. do you like it? then find a 34 or so tooth front that will fit your crankset and a rear wheel with exactly whatever sprocket worked, and remove the shifters, derailleurs, and cabling....
Experience is what you get when you don't get what you want.