Changing the rim and not hub is not worth the all the time it will take and a novice mechanic can screw it up altogether...
I suggest to get commuter tyres, something like 26" x 1.75 and hope for the best.
Apart from the drag from wheel, another drawback will be small cockpit of your 3700. You can fix end bars on the handle bar, that will give you some option of flexing your tired muscle on those 150 km rides. Aluminium bars similar to those in Hero Thunder comes for 100 bucks a pair from Metro Cycles.