My HR this morning was 49. Today is a rest day. I'm running three times per week. When I get up to about six miles, I'll level off at that distance and run it for a couple of months to build a base. Then I'll add a fourth day, starting at half a mile, and I'll slowly build up the fourth day to three miles. After a couple of months I'll add a fifth day and build it from half a mile to three miles. That will give me three six milers and two three milers per week (24 miles). After three months of building a good 24 mile base, I'll start my marathon training by extending one of the six mile days to a long run of 15 miles (33 miles per week).
After a year of 33 miles, I'll sign up for the St. George(Utah) marathon in the fall of the year. During the preceding summer I'll extend the 15 mile long run up to 20 miles and the other distances up a couple of miles, peaking about a month before the marathon. I'll just run the 20 miler one time. During that last month I'll taper back to 15 miles for the long run but will keep the other distances at their extended values. After the marathon I'll go back to the 33 mile base and will keep it during the winter. When spring comes, I'll start to work up to another marathon that fall.
This is the plan I used for my four marathons in New England and it worked well for me. I'm older now (68) and need more time for increases in distance and for recovery periods. Also, I'm running at a 5000 foot elevation instead of the 300 foot elevation of my former home in Massachusetts, and that means I need more time to build up endurance. The good news is that St. George is only 3000 feet. That decrease of 2000 feet means I'll be running the marathon with more oxygen than I'm getting now. Elevation does make a difference. A few years ago my wife and I visited our kids in New England and I ran a few miles in Connecticut. I could tell that I was running better than I did in Utah.
I'll probably be 70 or 71 when I do another marathon. Years ago when I finished my third marathon, I was happy that I'd finished it a few seconds less than four hours. While I was walking around recovering from the run, I saw an old man standing nearby. I asked him what his time was, and he said 3 hours 59 minutes and 50 something seconds. That was my time almost to the second! I asked him his age and he said 83. Ever since then I've had a goal to run a marathon under four hours at age 83. Right now I'm running four miles at age 68. That means I have fifteen years to pick up another 22.2 miles :-) A great goal to keep me going.
I don't plan to do any speed training while I'm preparing for my next marathon. It's risky, especially for an older person, to mix increases in distance with increases in speed. However, after I've done the marathon I'll do some speed work to bring down my time. I'm hoping to do one marathon per year. Maybe I'll get the world record for a marathon at age 100...