15 hr a week is great, sounds like a good mix of activity too. Is it getting easier or harder?51yrs now. After a repair(7yrs ago) of my BAV I jumped(4yrs ago) on up to 15hrs/week road cycling + swimming and light weight lifting. With a significant weight loss(90->65kg) I felt like a monster. Cycling involves mostly endurance FTP Zone times(z1-15%; z2-40%; z3-30%; z4-10%; rest higher); HR Zone mostly 2 a 3.
I have heard of a 72yrs old enthusiastic cyclist (mechanical mitral valve) making 30ths km/year, riding also in higher HR zones.