Exercise alone is not usually enough to sustain weight loss.
The more muscle you have the more calories you burn, but from around the age of 25 the body starts to lose 2% of muscle mass every 10 years which goes up to 6-8% the older you get, hence making it harder to maintain your weight or as popularly known as middle age spread.
Along with cardiovascular activity (running, cycling, swimming, walking and so on which is good for weight loss and overall health), resistance training your muscles (weights, pushups, elastic type trainers etc) will minimize or help to reverse any muscle loss, unfortunately none of this is any good without a proper diet regime.
Diet has far more impact on weight loss than exercise does but it's just as important to have an active lifestyle. Here are some interesting stats and a bit of fun;
Effort required to lose 1lb of fat from an 11 Stone person
An 11 stone person walking a brisk 4mph for 42 miles will lose 1lb
An 11 stone person running an 8 minute mile for 29 miles will lose 1lb
An 11 stone person cycling 12-14mph for 79 miles will lose 1lb
An 11 stone person having sex with average effort 79 times will lose 1lb!
I know I couldn't believe it either but please don't let that stop you exercising or creating an active lifestyle, it's important to our complete wellbeing and will enable us to enjoy our life well into our later years.
Another easy trap to fall into will be very similar to many people.
For example: this particular person went down the gym with her best friend, did a good healthy workout and then straight after went for a coffee and a coffee house style biscuit, thinking that they worked so hard at the gym a little treat wouldn't hurt. Bear in mind you can spend an hour on a treadmill, burn 250 calories and eat 300 calories on the way home at the coffee shop, or worse still, drinks at the bar.