Some thoughts...
Be certain that you have measured your former rate correctly. HRs in the 40s are quite low and rare, and would indicate extreme training, similar to competition bicycling. If your heart were pacing at, say, 44 BPM right now, they would very likely lower your Metropolol dosage to bring it up.
Your heart rate, your blood pressure and presence or absence of palpitations will come and go over the first year, predominantly the first six months. Your heart first recovers from the physical cut and stitching, then rebuilds the tiny, surface nerve paths that were cut, then begins to return to a smaller, more normal size, based on the reduction in its workload.
In general, it doesn't seem to pay off to react too quickly to one of these symptoms, as they tend to be transitory. Of course, if there is abnormal pain, shortness of breath, or dizziness associated with it, then it should be looked at immediately by a physician. Blood pressure in particularl can have bursts of being off-kilter, and it's difficult to determine if the treatment should be wait-and-see, temporary, or permanent. I tend to lean toward the first two, as many of these issues pass in hours, days or weeks, but you do want your physician to make that call.
If your heart were beating above a "normal" rate, it would mean you should revisit the cardiologist, probably for more beta blocker. Yours is not, so it is sufficiently controlled to get you through the recovery period.
The fact that your HR doesn't rise much when you walk is probably due to the Metropolol. Because it is holding back normal exercise sinus tachycardia to the 70s, I would not be inclined to believe you should seek a higher dose at this time.
Your heart should eventually return to a rate similar to its former rate. You will need to give it some time, though.
Best wishes,
