Don’t set yourself a daily schedule; it is far more sensible to run a weekly one, because you can’t tell what the temperature, the weather or your own condition will be on any given day.
You don’t know what the weather will be like on race day. You don’t know how you’ll feel. You don’t know what the temperature will do. If every time you encounter anything inclement, you fail to train then you are depriving yourself of many opportunities to develop mental and physical toughness. There are some runners I know (and I had a tendency towards this too!!) who will grab any excuse to postpone a training session. This is a charter for doing so!!
However, there is also a sensible side to the law. There are runners I know who become so obsessive that they will continue to push on no matter what. This type of runner is probably in the majority – most runner I know have at some time or another ended up injured because they were training when they should have been resting (and that’s a group that I can include myself in too!)
The sensible line is somewhere between the two. When I’m writing a training schedule for a runner, I will usually write the keys sessions for the week in detail and then give general advice about any additional sessions. This gives the runner the flexibility to perform those key sessions on the most suitable day. Part of that flexibility is also to choose when to take a day off.
So, I’ll agree with the general principle that setting daily training in stone can be detrimental, I would also add the caveat that training hard in poor conditions can prepare for racing in difficult conditions and can improve confidence for race day.
Train smart!