Master returning but very disappointed

[runkona]

Background
Now 56 with 4yrs off from running
Very deep experience in triathlon, running, cycling
Last 12mths very solid cycling fitness no running
Prior to retiring from triathlons, running 4yrs ago 5k 18:xx 10k 38:xx HM 1:24:xx 26.2 3:12:xx
Vo2 running was 48
running treadmill for 4-5 weeks now, 3 weeks long runs now up to 2hrs about 13.1miles/21k
I have from time to time run all out 800 on grass at 2:49mins/ 3:30mins/km,
This week started Cv - Oh boy what a shock to the system and how the body aging and specific exercise has greatly affected speed
My max Hr running is 163
I could not go any faster than 5:15mins/km or 11kph pace for 20 mins what I thought was my tempo pace the Hr was 138 on the 1st 1k , the 4th 1k was 145.
5 yrs ago I could go 6:20mi/mi pace with ease at Hr138

Any else over 50 with similar stories coming back to running and feel where is the speed even going to come back!?

My plan is to go back to triathlons in Australians Summer starting October'22 in the mean time I have 6-8 weeks to get race fit for Duathlon and hopefully re-capture sub 20:xx 5k. The journey will be interesting to look at my physiology adaption and to see the effects of aging on speed and how quickly a somewhat above average fitness male can gain speed.
My plan is trail run on the easy days for an hour average Hr is around 120; CV Tuesdays Tempo Thursdays and long run 13.1mi/22k at comfortable 138 Hr, Saturday is my 3hr group B bike ride 1st 90 mins is high 140 Hr 2nd 90 mins is on the back of a bunch peloton in Hr recovery mode with occassional short sprints. Friday & Monday is usually a rest or very easy.
For 4 mths I had been also cycling during the day 2hrs letterbox delivery for my own real estate work but while it is slow I am doing some physical labouring work to keep the cash coming in. The body is really physically tired by the time in the evening to run.