Half marathon training - 4 to 5 mile dip

I've been running properly since April, 4-5 times a week.  Mainly 5km, the odd 10km and am on week 3 of training for the Worksop half.

Current training consists of approx 5km on Monday, long run on Tuesday (currently up to 9 miles in 80 minutes), rest day Wed Fri and Sun, hard session with club on Thurs which involves either hillwork, interval training, speedwork and a steady 5-7km on a Saturday.

On my long run on Monday this week my training partner who I regularly run with has noticed I start to get tired at approx 4-5 miles in.  I slow down for 0.5-1 mile and then have a second wind and am fine for the rest of the run home, often opening up for the last mile..

Any tips on how to stop this 4/5 mile dip please?  My training partner (ex PT) suggested breathing in through my nose, hold for a split half second and then breathe out through my mouth for 20-40 strides every now and again to get more oxygen in my muscles which worked nicely, and squeeze my core to change my striding to more mid-foot as I tend to heel strikel, particularly when tired.

Any advice greatly appreciated