Don't stress too much over your HR zooming up so quick to start. I have the same thing and was tested at a lab and the doc reassured me that it was just the way my body perceived the need to get up to a fast HR then settle down if needed. However he did say to stick to the correct zones that they worked out as I did hit MHR but the end of the test. Money well spent but a very hard workout to do whilst wired up and stuff to machines.
I am still dithering around and not entered anything yet. I feel reluctant to spend money and then not be well enough to complete the event. This last round of bugs has set me back totally to square one again. I went swimming for 30 minutes last week and that wiped me out although it did seem to help shift the bunged up nose.
I finally got outdoors to do some exercise on Sunday. 40 minutes very gentle bike ride with Mr Steady. Then in the afternoon a 4 mile stroll with friends and dog. Tired afterwards but the cold is still hanging around and at my annual asthma review I was told to not exercise until well and rebooked to see the nurse in 6 weeks.
The only good news is that I have not put any weight on, but that will be due to a frequently disappearing appetite.