How long to rest between long runs?
Now for starters I want to clarify that what a long run actually is will be very subjective. For me, since I haven't really been training for running for very long and only recently transitioned away from strength training a 10 kilometer run is something I would consider to be a long run. There are people I know that do regular marathons though and for them a 10k would be a short run. I admire that but at this point in my life that seems very far in the distance... just like the finish line for a 10k.
As far as "rest days" are concerned this is also very subjective. For me at the level of endurance that I have already accomplished, this would mean that I go for a long walk or ride my bicycle a bit - like an hour or so - at a medium pace. While this isn't technically resting, it doesn't really tear me up like running does, so I don't really consider it exercise.

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According to science, if you have been training for a while and achieved a certain level of fitness that you can take weeks off of running just before an event like a marathon and hardly even notice any sort of drop in performance. However, for people that are looking for their own optimal performance in these things that completely taking time off for more than 3 days would be excessive.
The experts have various ways of telling people to do this, and since I am relatively new to the process I am taking the lower end of what they suggest... But all of the experts such as trainers seems to be in some sort of agreement about optimal performance and how the "best" thing to do just before a race is not to just sit on your ass and do nothing, but to do something slightly different than what you are training for.
This seems very contrary to what one would think works, but for me it was worth investigating.
For example, when I run a 10k at what I consider to be a very "doable" pace, I will set out and try to just remain at under 7 minutes per km for the duration. After months of doing exactly this, I was able to do it with relative ease. However, this was NEVER true if I tried to do them on back-to-back dates. In my own experience if on my "day off" I went out there and instead ran a faster 5k then just walked the other 5k, that the following day I would be able to easily nail the sub 7 minute KM pace. If I tried to run consecutive 10k days, I found that my performance struggled and if I did manage it, it was very difficult for me.
This seems like the opposite of everything we think we know about fitness (or perhaps it just does like a normie like me) but if you think about it, it is kind of the same as the rules we have in strength training. You don't do chest day every day do you? Well what if you tried? In my experience if I did do chest 2 days in a row I found that I was considerably weaker on the 2nd day and while some of this is definitely psychological, a lot of it is likely because I didn't allow my muscles to recover and grow.
I tend to think about all muscles, whether they are "life heavy rock" muscles or "run really far" muscles as being things that need some time to grow before they are going to be more effective the next time.
Running is different though, and according to one particular sport science running trainer named Dr. Robert Reiland he suggests that if you are the type of person that gets stir crazy and feels like you MUST train every day than to mix it up and do some circuit training and the days leading up to your race rather than practicing exactly what the actual race is going to be.
In his suggestion he stated that it was good to warm up at a pace that is about 30% slower than you intend to run the day of the event, but every km or so to run a lot faster than you intend to run on that day. Call it interval training or whatever you want, but on the bits where you will be running much faster than you normally would these should only be 100-200 meters, then return to your much slower pace.
He sums up the end of his explanation that is less science sounding and more layman's speech
Think about what your goal is and adjust the training accordingly. If you’ve been averaging say 8 miles or more a day for months, jogging 2 to 3 miles a day during tapering will feel like doing nothing while it will still give you the sense that you are continuing to keep your legs moving.
And for me this is something that I don't mind doing and while I am not a scientist nor do I claim to have answer to others, it has been working for me.
If I do a 10k at a 7 min / km pace on Monday, I will run a 5k at 8 mins per km on tuesday with 100 meter sprints at something crazy that I cannot possibly maintain like
5 mins/km. This obviously is much easier to accomplish on a track and fortunately I have access to one of those near where I live.
This post isn't so much as something to tell all of you what to do so much as it is something that I am doing that is working for me. I am in my late 40's and have spent the past 20 years focusing almost exclusively on strength-training. Cardio is still relatively new to me. So if you have a program that you have used and is effective for you, I would like to hear what it is.