When I was younger and ran marathons in the 1980s, I ran five days per week. I gave myself a rest day before my weekly long run and my usual rest day on Sunday after the long run. I say "usual rest day", because I consider Sunday my Sabbath, and I never run on Sunday.
However, after I recovered from my auto accident in 2004, I ran three days per week. Every so often, I would add a day, but I reverted back to three days per week because I could tell my body wasn't ready for the new day. Finally, one day I added a fourth day, and my body told me it was ready for that increased stress, and I continued running four days per week. But, in January, 2009 my running was stopped by blood clots. After five days in the hospital, I started walking and then running three days per week, and I'm still doing that number of days (or less) per week.
When I tried to run an additional day each week, here is the procedure I followed.
- I capped my three days of running at the values they were at. Typically, I had one day as a long run, one day as a medium run, and one day as a rest run. The long run was usually around 5 or 6 miles. I capped the three days, because I wanted increases in distance to be due to the new day.
- I added a fourth day of running and limited that day to about 0.5 miles. I placed the new day around my rest run rather than around my long run.
- I used the 10% rule to increase the new day to my desired value for the new day. My fourth days (and fifth days) were always rest days, following the weekly strategy of one long run, one medium run, and the remaining run rest days.
- Once the new day reached my desired value and my body told me it was getting used to the additional stress, I began increasing my other runs until they reached their max values.