Things to check:
1. Tire pressure. You probably have 27" tires that should be inflated to at least 90psi. Use a good floor pump with a pressure gauge.
2. Bearings - check wheel bearings for play that would indicate they need servicing. Grap the top of the wheel anf firmly shake from side to side int he frame and fork. If you can feel rattling or a clunking then the bearings may need service. Do the same thing with your cranks to check for bottom bracket play. THen take the chain off and sit it on the bottom bracket shell out of the way of the chainrings and see if the cranks can be spun without much effort. If you spin the cranks and let go they should take a few seconds to stop, and you shouldn't feel much resistance at all. If you hold the bike up and spin the wheels they should spin for quite a while unless the brakes are rubbing.
3. Make sure brake pads are clear of both sides of the rim and not rubbing (although this seldom causes any major loss of power).
Otherwise it's all you, buddy.