All good advice up there I think! I do not think that I need to invest in a heart rate monitor but it may be advisable that you spend some money on buying one. Even though the medical advice given to you is to do more cardio but cardio could(it might not but it is better safe than sorry!) cause another problem or make the problem worse. If you are in doubt make sure you talk to your doctor, they are the only people who may be able to advise you on this fully, like I am only going on an assumption as when we work hard our heart beats faster and harder😕
Indoor cycling is the most boring thing in my opinion because our scenery is not changing. But, if you are also cycling the same route over and over this can get nearly as boring as indoor cycling, so have different routes known so you can change things up a bit. Full on mountain bikes are more generally aimed towards mountains because they can be quite tough to cycle fast on the road due to the increased friction involved, but if you are looking for something more recreational this may not be a problem in your case, but as others said there are bikes which are more universal.
Ya, do go and look and in your local bike stores. They would be the best to advise you. Especially, because they will have knowledge on your local area and it's terrain and maybe even discuss routes with you. They may also fit the bike so you are at less risk of injury and if the bike is the right size you would be able to cycle more efficiently. If you have any more questions, please ask away!😉
Good Luck,
CycleLad99