Key takeaway: Use Consistency to adapt
Some serious running books with relevant chapters:
- The Science of Running by Steve Magness
- Daniels' Running Formula by Jack Daniels
- Build Your Running Body by Pete Magill et al
- The Lore of Running by Tim Noakes
Standard training programs try to minimize injury using consistency and gradual changes. By training on a near daily basis, creates a much more regular routine for your body, and building up mileage slowly also helps your body adjust.
Muscle building is very adaptable. You may be sore in the days after you run, but if you drink and eat well, muscles should recover quickly. Building up mileage will help your muscles prepare for longer distance runs. If you strain a muscle (possible if you run while you are sore and/or take a bad step), you might need a few days of rest. 2 days off might be enough! Muscle recovery slows drops off quickly after your run, so recovery 2 days after a run will be significantly slower. A short run (just 1 or 2 miles will work) on a day off has been shown to improve recovery.
Tendons and Ligaments
These are not quite as adaptable as muscles, but providing muscle-bone and bone-bone connections, the running will add stress to them. They will slowly adapt over the course of months, and regular exercises and stretching can help loosen them. Common issues like achilles injuries crop up when training is adjusted too quickly. Tears are unlikely, but even soreness can hang around for weeks.
Bones can support a volume of stress over time (maybe 20 miles a week of running), but are gradually damaged by repeated use. If you overwhelm your bones, you can have a minor contusion or a fracture, with recovery times from 2 weeks-3 months. With a fracture it is likely you will not run in the marathon you are targeting. The best protection is to slowly build the volume you run each week. Running many days allows you to build high regular volume over the course of training. Many runners will run quite a bit farther (20 miles) in the weeks leading up to the marathon. If your running volume is 25 miles normally, and you run 35 or 40 miles 2 weeks before your marathon, you are much higher risk for a stress injury. By building large regular volume, you can avoid the big jump.
Conclusion: Reduced Injury Risk
In general, running more regularly reduces injury risk, even if some runs are short. This doesn't mean you will get injured if you run inconsistently. You are also not invincible just because you are consistent. Consistency doesn't protect you from a bad step. But consistency reduces risk.