[ratter]

Assuming you have knobbys on the bike still, if so replace them with mtb slicks when you get the bike serviced, it will allow the bike to roll easier on hard surfaces and feel smoother, also keep the tyre pressure high to help eliminate some rolling resistance, you may need to purchase a floor pump to do this though.
Keep your cadence (leg rpm) high, most beginners use to low a cadence and that's bad for knees and takes the strength from your muscles quickly.
Have the bike set up for you, correct fit is important to comfort and strength, there are plenty of websites that will give good info about bike fit, there are professional people out there that can fit you up, but it may cost more than a cheap bike with some of them, but people swear by them.
Find people in your area to ride with to keep you motivated.
A bike computer can help you perform better as you can track your ride info and challenge it to do better.

Good luck