One question you must ask yourself is: what is your aim? What is it you are wanting to achieve?
The body is an adaptive machine. When you demand something of it, it will do its best to giv it to you, but it'll take time (as we all know). So if you want it to be able to run, you run and rest and the body works at getting better at running. If you want to build strength, then the body will work on that. If you do both on the same day, then the body will work on adapting to the demands you placed on it last
, unless there was quite a period of time between the two workouts (i.e. running in the morning, strength in the evening).
If you can, I would recommend you run on days when you don't do your strength training. If this is not possible, leave as much time as you can between those workouts, and it won't matter which one you do first. The time between, allowing for that body adaptation and recovery, is the important thing.
As far as how you split your strength training; I like a split routine, but sometimes I like to focus on one bit of me that I feel needs particular attention. Again, this is largely your choice and is determined by your aims.
Quite some time ago, during an AMA
, I asked a similar sort of questions of @Damer
and here is some of the answer:
"This, then leads us to your question: How often should we train and what kind of gap between each type of training are we looking at? Obviously, as you suggest also, part of the answer depends on the level of fitness of the individual. But if we assume that this is not the issue then the gap between training sessions will depend on the intensity of the session itself. Suppose, I run 10km in the morning at 40min flat. And then in the evening I do weights to failure. The chances are that A. I will be too tired in the evening by my super-fast morning run to do much and B. The interval I have left for recovery between the two training sessions is insufficient if I did the time I did in 10km.
A study that dates back to 2008
suggests that there is a ceiling to intensity beyond which no further benefits can be derived. So, if you train for an hour flat-out then upping that to two hours will certainly tire you out but it will not give you twice the benefit you expect. This ceiling to intensity gives us an idea of recovery-time windows. So, if you were to run (for cardiovascular fitness) in the morning you could do strength training in the evening and still get benefits from both forms of activity provided that A. Your running was not super-intense and B. You had plenty of food and 'rest' before you got to the second session. Remember this is all about energy management. That study suggested that 2 - 3 times a week strength training with running in between (or the reverse) produced good results for both.
This interval can be significantly reduced if the workout intensity is lowered. A 2018 study
where the workout intensity was constant showed that even when the training is specific (i.e. only strength or only cardiovascular fitness) the inadequate rest leads to decrease in performance not an increase. This had also come up in an earlier study
that looked at strength training in older adults and found that two-three times a week was more effective at building strength than say, ever day. So adequate rest is important and/or as we often do with Darebee programs, we vary the intensity from one day to the next and reduce it to minimal levels during "active rest" days so that you can keep on gaining day after day.
Finally, the last part of your question is a variation of the first half. If we throw more goals in the mix: train at the same time for speed, strength, endurance etc what will happen? Well, without sports or movement specificities in mind we end up achieving little of what we expect as our body faces competing demands for the energy necessary to power the adaptations we seek. If we are sports-specific however (football, rugbly, martial arts) then the training we do is geared to that and it doesn't matter a lot if we pack everything in. The chances are that we are training hardest for what is most important in the sport and no sport demands 100% output in every physical attribute. Martial Arts, for instance, demands we are flexible, fast and strong but flexibility and speed trump strength (because it's power that counts) so we end up doing less strength-orientated exercises anyway. "
I hope this helps.