There is really only one thing worth knowing here, and it is a gentle preference, not a rule. This page explains it, and is honest about how much the evidence actually supports.
Where you can, try not to strength train the day right before your most important hard run.
That is the whole thing. If your key session that week is intervals or a threshold run, arriving with fresh legs helps you get more out of it. Lifting hard the day before can leave you a little flat for it.
This is a soft preference. It protects the quality of that one run. It is not a safety rule, it will not cause injury, and it does not harm your long-term progress to arrange things differently. If your week does not allow the ideal, place your session where it fits. It is a preference, not a rule, and an imperfectly placed session is still a good session.
After a strength session, running economy (how much oxygen a given pace costs) is slightly raised for a few hours and returns to normal within about a day. Harder, maximal running seems to take a little longer to feel fully sharp again. That is the reason for the preference above: it is about protecting your hardest running, not your easy running.
Honestly, the studies behind this are small, mostly in male runners, and used heavier gym sessions than the ones in this app. So the real effect for most runners is probably modest. We would rather tell you the one thing that holds up than dress a coaching hunch as settled science.
Your long run may be the most important session of your week, especially in marathon training. But important is not the same as high intensity, and the evidence here is specifically about high-intensity running. We do not have good grounds to tell you exactly how to place strength around your long run, so we do not pretend to. If you would rather keep the day before your long run clear too, that is a perfectly reasonable personal choice.
Sometimes a hard run and a strength session have to share a day. If they do, doing the run first and leaving some space before you lift is a gentle preference. We are not going to give you a number of hours, because the research does not actually establish one.
Strength for Runners simply asks which weekdays you would prefer to strength train, and leaves the choice with you. You know your week better than any algorithm does. Keep the principle above in mind when you pick, and remember it is a nudge, never a rule to fail. And whatever you choose, if a session ends up on a different day than planned, it still counts.
This is general training guidance, not medical or individual coaching advice. How much you can train, and how much rest you need, is personal. Listen to your body and adjust.