Our expert says:
Apologies for the delay in replying - I have been at the Comrades away from my normal routine
To answer your question, I think it's important to understand why you gained so much weight so quickly, because that does not just happen. So whether it's related to a metabolic problem (like your thyroid), or diet, or a change in your routine, there is something different now compared to about a year ago. And seeing a doctor to check out the thyroid and other metabolic syndromes is a starting point.
Once that's done, it may be treatable. Exercise is still a good idea. My advice is to walk, as you propose, but don't worry too much about the heart rate. If you can walk 5.5 km/h, and you still FEEL OK, then it's fine. This theory that there's a maximum heart rate is not really true - as long as you are comfortable and able to exercise for 35 minutes, then you will be training at more or less the right intensity anway.
Finally, you can check out the walking programmes on the Health24 site - they are under "Programmes" on the Fitness page, I'm sure you will find one suitable for you. If you are already walking 35 minutes, then I'd take the 10 km programme and start in the middle somewhere.
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