I am also 5 months post op, age 61. I'm doing 51 minutes of cardiac rehab 3 times a week; 5 minute warm up, 20 minutes each on the treadmill and exercise bike, and 6 minute cool down. On the treadmill I am up to 2.5 mph with a 3 incline. I started out at 0.6 mph with 0 incline. The nurses gradually increased my speed and incline. If you are on a treadmill, I recommend adhering to warmup and cool down time. Yes, it's very boring walking at 0.6 mph, but it helps with stamina.
The exercise bike is what wears out my legs. As the level of resistance has increased (up to level 8 now), my pedaling has slowed down so I'm not so quickly worn out. The nurses said this is OK. Try slowing down your pace from 2.8 to maybe 2.5 with the warmup and cooldown.
Main thing is, you have got to build up your exercise routine gradually, even if it seems painfully slow as it does with my rehab. Remember, you are still recovering so baby steps.