Replacing runs with cross trainer sessions after back strain

rockinrockyrobin

Have been training for my final HM of the year on 4 runs per week with the race on Sunday 3rd, so was planning an easier week in the build up anyway

Training has gone well and then on Tuesday night I bent down to pick something up and felt a small tweak in my lower back. Thought nothing more of it and went out for my run on Wednesday morning but have had a couple of days of real pain and lower back stiffness as a result, uncomfortable to sit for long periods and then very cagey when standing up

Now trying to strike the balance between being well enough to run the race but on the other hand not leaving it 10 days before I run again

I'm fortunate to have a cross trainer at home and therefore did a session on that this morning to replace my Friday run and didn't suffer any discomfort while exercising but not sure whether 40 mins on the cross trainer replaces my usual run.

I guess my question is how would I replace/replicate the same effort of a 7-8 mile run (at around 8.5-9min miles) with an equivalent session on the cross trainer, should I have to skip further runs in the coming days

Hoping to be back running before long, I'd be due to run again on Saturday and then Monday night, Wed morning and perhaps another on Thursday prior to 2 days rest before the race