Though Now 70 I Am A Lifelong Athlete. I've Run

Though now 70 I am a lifelong athlete. I've run 73 marathons and completed 17 Ironman contest, plus 100's of shorter races. Being fit is absolutely core to my being. Four years ago I had to stop running for a year because of a sciatic nerve problem. Took awhile to rehab but that seems under control now yet my cardiac capacity is way down. I fully accept the impact of age but I am not willing to accept this level of fitness. My diet is pretty much ideal, I get lots of rest, and my resting HR is 48 but my BP went up last year to about 170/100. For two months been taking Lisinopril 10 mg and BP is down to 130/78. But when I go out to run my HR spikes almost immediately. I never ever run without first warming up either on my bike, rowing machine, or swimming so my body is ready to run but my heart seems not. Do you have any suggestions as to test I might seek or anything I might try? I am extremely motivated to be a runner and triathlete for the rest of my life so I will do just about anything to be what I believe I need to be. Thanks for your time.