So my tutor and I have been talking about this example and I still can't ascertain an answer I understand from him as to why it works (the piece in bold).
ただ走るのではなくて サーかを しながら走るのがいいです
What I understand is that the の acts as a noun connecting piece, just as こと would. What I don't understand is how he has connected 走る with ではなくて. Is this the latter a noun? I really don't understand its use here. Why not say 走っていないけど？
So, from your comment it seems like you more or less understand nominalisation. Let's work backwards from the translation.
ただ走るのではなくて... means "rather than just running ...".
走る is the verb meaning "to run". We need to turn this into "running". To do this we nominalise it by adding の. If your uncomfortable with thinking of "running" as a noun then think of 走るの as meaning "the act of running". That's definitely a noun phrase.
Next we have noun+ではなくて... this literally means "not noun and ...", but that's a useless translation in this context. Quite often ではなくて translates more naturally as "rather than" and that's exactly what it does here.
Finally, your alternative 走っていないけど... would mean "I don't run but ..." which has a rather different feel to it.
The の in「ただ走るのではない」is a normalizer. In other words, の turns this verb-phrase 「ただ走る」into a noun. To help you to understand「ただ走るのではない」, it translates to "just-run-thing not" or "it is not only run-thing."
