I have now done a number of runs using only nasal breathing. The first was 7.5 klm at 8:00 mins per k. Then a 5k at 7:30. Then a parkrun at 7:15 and today a parkrun at the same site at 6:45 per k. I am going to continue this over the next couple of months - will do easy longer runs around 7:30-8:00 and will do parkruns as fast as I can without having to resort to mouth breathing. My 6:45 mins effort today finished up quite hard for me ( the last 1K is uphill!!) and I almost had to gasp for air through my mouth as I finished but managed to contain it. At this time , I can't imagine being able to run any faster than this using nose only - but I think that it is early days and perhaps it is one of those things that improves with time and training. It does seem to give me a good feeling at the end of the run and recovery is extremely quick. I can of course run faster when using my mouth - but I have nothing better to do right now training-wise so I will keep on with this and see if it leads anywhere.