If you're 61, pushing for a heart rate of 180 is ludicrous in the first place IMHO. While there's quite some variance (+/- 20bpm IIRC) for predicting a safe maximum heart rate, a reasonable formula is 206.9 - (0.67 * age).
In your case this gives a maximum of 166. I'm not a doctor but my understanding is that there's not really such a thing as hypotension in someone who's healthy, provided you're not passing out every time you stand up. I'm 24 and it takes me a significant effort to reach 180 on a treadmill (the sort of I'd-better-stop-before-I-throw-up kind of effort).
Low blood pressure is probably accounted for by the vast amount of walking you do.
Source:
Maximum Heart Rate