Re: intervals. I’m 70 and try to do interval training twice a week at the gym. During riding season I do them on the elliptical, and off-season I use a stationary bike- both low impact. It’s my understanding that along with losing strength as we age (a reason for weight training), we lose aerobic capacity (VO2 max). Because of their intensity, intervals help us address that. I’m not doing anything outrageous: 20 minutes of 1:1 intervals.
Ive found it helpful to periodically schedule a session with a trainer who lives near me - a triathlete who is in her 60s.