When starting out with physical activity:
Interval training has some good evidence for improving fitness levels as well as muscle tone. The idea is that you do short bursts of vigorous exercise, followed by ‘intervals’ of easier exercise. It’s aerobic exercise we’re talking about here – so things like running or riding on a bicycle are perfect for interval training. The key is to switch up the intervals during the session, and also between sessions. For example – you might try jogging for 30 seconds and then walking for 45 seconds, and you might do that 5 times. Then you might jog for 40 seconds and walk for 30 seconds, and you might do that 5 times. On the next session you might try jogging for only 20 seconds but at a greater intensity, then walking for a minute, and do that 5 times, followed by a slow jog for 50 seconds and then a walk for 30 seconds, and do that 10 times. As long as you’re changing it up and slowly making it harder, you’ll see the benefits.
The National Physical Activity Guidelines for Australians outline the minimum levels of physical activity required to gain a health benefit and have great ways to incorporate physical activity into everyday life. You can find a copy of them here.