The approach has to be in the following order:
Figure out what went wrong. What you experienced is not normal. It could be anything from bad dehydration to a latent neurological or cardiovascular problem. It might even be dietary or simply a really bad flu. You'll be working with your doctor on this. I presume you are writing your question some days or even weeks after this event. The fact that normal strength hasn't come back is not promising.
Find out what you can do. Once the problem is diagnosed, and you understand cause and effect, it may limit your options to the types of exercise you can do. Again, this is something you have to discuss with your doctor once you know what happened. The key is to avoid the same events that caused the problem to begin with.
Come up with a plan that is slow and steady to get back to where you were, or whatever your new goal is. The focus should be on slow progress, and monitoring how you feel.
Unfortunately, without further information on why you had that incident, it will be very difficult to help come up with a plan for recovering that level of cardiovascular health.
One approach that can help you get back, slowly, is:
- Walk every other day. Walking is low intensity.
- The first day you go for about 40 minutes at a comfortable pace.
- If you can't handle 40 minutes at first, build up to that by maintaining the same pace but adding as much time as you can handle each day until you hit 40 minutes.
- The every day after that you try to cut a little time off the same route.
- When you can get the same distance you did in 40 minutes at a comfortable pace to a quick paced 25 minute walk, you can start introducing interval training.
With the interval training, again start light and build up. Perhaps 2 minutes walk, 30s jog, and do that for 20 minutes. Then start lengthening the jog time to 1 minute. Then start shortening the rest time to 1 minute.
At this point you can do any Couch to 5K, and build up your endurance from there.
While I'm a firm believer in doing strength training as well, I highly recommend you get the cause of the incident sorted out as well as you can. You might have to be very meticulous about the records you keep for a while. Keep track of what you eat, how you feel before, during, and after exercise. If you feel nausea start to come back, stop what you are doing. Put that in your notes. Review those notes and figure out if there's any correlation between those episodes and anything you ate, or how you felt leading up to that event.
The purpose of all that tracking is to help you recognize patterns so you can avoid future incidents like that. Even if the doctor seems to be less than helpful, meticulous note-taking and reviewing can help you find your own cause and effects. If a certain type of food seems to be associated with most of your incidents, try eliminating it from your diet for about 3 weeks and try to introduce it back after that period of time. If the symptoms return with the food, there is a good chance you might have an allergy to one of the ingredients. 3 weeks is enough time to completely purge any lingering affects from any food. If the symptoms return before that 3 week period is up, the problem is likely someplace else.
If nothing else, sharing your observations from those notes with your doctor may help them help you.