Hi hannah,

I always work out on FDs and don’t usually have any issues with lack of energy. As you are fairly new to 5:2 it might take your body a few weeks to adjust. Once it has there is no reason to not do things as your body will be efficiently switching to using fat stores for energy.
Is there any possibility that you could work out early morning on FDs. I don’t know what time(s) you eat on FDs, but it would be much easier to exercise early in the day.
I only eat in the evening on FDs and on the rare occasions when I really need something after the gym I have a whey protein drink. I found some sugar free vanilla flavour in a wholefood store and made up with unsweetened almond milk and a tiny bit of Stevia it comes in around 100 calories, tastes fine and does the trick.

you will get maximum effect from exercise when fasting.

http://fitness.mercola.com/sites/fitness/archive/2013/09/13/eating-before-exercise.aspx is just one of many you can find.

I find that short, high intensity weight and resistance gives the biggest bang for my buck. Max weight low repetitions target each area of the body 20 minutes to half hour and I’m done. Does it help? I don’t know, I do have more muscle mass and flexibility, at least I think so.

Due to COPD, heart failure, and heart disease high intensity aerobic exercise is not possible, ie. bike, tread mill, stair climber etc.