You may want to pick up a copy of Joe Friel's "Cycling past 50" book and a Heart Rate Monitor. The HRM is not to keep your heart rate in a "safe zone" but to help you guage your output. Set some goals but don't be discouraged if you don't quite make them. Keep at it - you will get stronger quickly.
Put slicks on the MTB - it will be much better on the road, wait for the road bike purchase until you have been riding a few months and have a better idea of what you want.