Today is a rest day. My wake up HR was 50. My wake up HR has been pretty consistent during the past week. I think that's because I've been consistent in getting close to my seven hours of sleep. I've been measuring my wake up HR for almost the full 30 years that I've been running, and HR is very sensitive to sleep. Last fall I was only getting 5-6 hours of sleep, and my wake up HR was 55 or higher. As my HR goes up, my endurance and energy level go down. This is probably to be expected considering my age (68), but the same thing happened when I ran marathons at age 45-46.
I'm currently running 3 1/4 miles of long slow distance (LSD) at 9 or 10 minute miles, and my HR doesn't vary much during the run. I'm working up to 6 miles three times a week, and I'm not doing any speed work while I'm increasing my distance. Both increases in distance and speed increase the stress on my body, and I'm controlling the amount of stress by changing distance or speed but not both together. After I get up to the 6x3 distance and run it for a couple of months, I'll add a fourth day starting at 1/2 mile. When that day gets up to 3 miles, I'll add a fifth day and take it from 1/2 mile to 3 miles. At that point, I'll be running 6, 3, 6, 3, 6 (heavy/light) for 24 miles/week. At that point, I'll begin my marathon training and slowly increase my 5th day up to 15 miles. After I reach that point (33 miles/week) I'll level off and run that year around. After a year or so then I'll begin training for the St. George marathon and work during the spring and summer to get up to 45-50 miles per week, extending the 6 miles to 10 miles, the 3 miles to 5 miles, and the 15 miles to 20 miles (just one time for the 20 miler about a month before the race).
When I level off at 33 miles per week, I'll begin to do speed work, but I'll only do heavy speed training once a week (now that I'm older, maybe once every two weeks. It depends on how fast I recover).
This plan for marathon training may not seem very exciting to you younger runners, but It's the plan I used in running marathons back in '81 and '82, and it's my way of running pain and injury free. It's a slow plan, but it works for me.