Training after long lay off

At age 50 I was an incredibly fit athlete, x-training bicycling/weight-training. Then I got hit with myo-carditis, a viral infection of the heart. It nearly killed me; and for the last 4 yrs I wished it had…..I’ve been completely beaten down. Then, a few weeks ago, I decided to get my bike out, cleaned it up, preped it for a ride, and went for a short ride. It felt good…..really good. I decided to get on-line and see if there were any biking forums out there that might be helpful, and stumbled upon this one. Although myo-carditis is fairly rare, I found someone who had recovered from it (and it’s resulting damage) and had made a fantastic recovery, was riding again, was racing again, was living again. And believe it or not, it was like a light went on, lighting a darkened room. Why I had given up, I don’t know. And why this one feedback broke this terrible depression, I don’t know. BUT IT DID!! THANK YOU. THANK GOD!!
OK, enough of that. Now for the training advice….and I REALLY need your help here…..PLEASE?!? I was told by my doctor that because of my age (54) and sedentary habit’s the last four years, I must keep my training very light…..until I get “in shape.” I have been riding every other day for the last two weeks: the first 3 rides were 45 minutes; the next 3 rides have been one hour each, with a 10 minute cool- down. My doctor said I should keep my target HR at 115 - 125. I’ve had a HRM only for the last 2 rides; but, I find that at a comfortable pace I average about 130 (last ride High=167 for about two minutes up my first small hill, Average=132, Low=92. My resting pulse is around 60. Here’s the thing though: I do not get winded (out of breath) at all with these heart rates. Does this mean I am in better shape than I (and my doctor) thought. I have not had any type of stress test in four yrs. Bear in mind that I am on blood pressure medication (50mg Toprol XL once per day) which maintains my BP at avg. 130/80. (I have a high-stress job). Do you think I should I accelerate my HR during my rides, occasionally, to the point of getting slightly “winded” ?? When I had my last heart-cath four years ago, there was no artery disease….I had very little narrowing for a man of 50. Is it possible that even though I have had a sedentary life style for the last four years, that because of the shape I was in prior to this, that my target zone(s) are higher than what the statistics recommend. Or, am I not getting out-of-breath because of the blood pressure medication? I guess I’m just not used to NOT “sucking air” like I used to. Is getting out-of-breath just too dangerous at 54 when you are just getting back in shape? If I sound like I’m confused, I am. Help.