Thanks Terry for your help
Only just started exercising again, for the last month and a bit while not exercising it was high protein and vegies, low fat/sugar/carbs. Now that I'm back exercising my plan is 3 running a week, 2 gym.
Last night I went for a run, did 2.64 km in 15:30 minutes. The start was at a normal run but my heart rate reached 175 in five minutes and to keep it from topping that (I didn't want to go over as I believe around 165-170 is the max of my exercise 'zone') I had to jog almost at a walking pace at times. After there was no endorphin high, just mental exhaustion.
Does that kind of heart rate increase seem steep? It took me ten minutes of walking to get it back to 145.