Unfortunately, as you get older, you lose muscle. And muscle is such a great fat burner that you really do have a harder time losing fat if you're also losing muscle. If you are doing the same amount of exercise and you haven't changed your routine as you get older, even though you are exercising, you still can get fatter over time. Loss of muscle decreases metabolic rate, which means your body is burning less calories, and since eating doesn't really change as we get older, that also works against us.
Consider this- the muscles burn 30- 50 calories per day per pound of muscle. Once you hit your early 20's, unless you continue to increase your weights, you begin to lose about half a pound of muscle per year. If you did the math, think of this: losing 25 calories per day times 365 days= 9125 extra calories the body isn't burning= 2.5 pounds gained per year. Now, that's just for the first year you've lost half a pound of muscle. If you continue to lose muscle over time, you end up gaining more weight over time... and all this why you continue to do the trusted exercise routine you're used to doing and wondering why you continue to exercise but gain weight. Potentially, in 5 years time, you could easily gain 12.5 pounds. In 10 years time, you've gained 25 pounds. If you don't do anything to increase muscle gain and metabolism, you could be well into your 40s and 30 pounds or more overweight.
Besides that, you have to really regulate what you're eating- if you exercise a lot, it can increase your caloric intake requirements. By eating too little, you work against your metabolism- your metabolism will slow as your body conserves energy to complete its activities. That's no good.
So, I would suggest:
1) Increasing your weights. If you already have a weight routine, it's just a matter of thinking progression. You should always work to increase your weights.
2) If you don't have a weight training/resistance training program, then you'll need to find a good personal trainer to work with. As you are 59, try to find a personal trainer that has a certification for aging exercisers (I have one, so I know it exists). Tell them what you told us, and tell them you want them to plan you a training program that incorporates strength training and works towards maintaining and/or increasing lean muscle mass.
3) Get rid of that OptiFast crap. Find a good dietitian and pay them to put you on an eating schedule. Be sure to bring in your training program so she can see what your caloric requirements are for the amount of exercise you do, as well as ensure that you still meet your fitness goals without slowing down your metabolism.
Good luck. Report back if you do these things and let us know how things work for you.