I've done the 5:2 before , and I'm not doing it again, it truly is amazing - for me its the holy grail of diets.
I exercise on fast days, when I did it previously I used to do two 1.5 hr runs every day (one morning, one evening) and rode my horses, was on my feet all day.
I've never suffered any ill effects, I always have so much more energy on fast days and my exercise routine seems easier on fast days vs non fast days.
I've just started the 5:2 again in the last few weeks, although I'm doing every other day fasting.
I don't run three hrs a day anymore, but I do 1.25 hrs of yoga, the 30 day shred and 3hrs of riding my horses per day, plus a lot of walking, mucking out etc.
I haven't had any I'll effects this time either, and I live in a pretty hot place too. Obviously YMMV, but fasting isn't the death knell for workouts 