I'm surprised that you'd risk feeling weary and a tight achilles with a 5k session so close to the start line, Skinny. Sounds like the sort of stupid thing i'd do rather than the sensible swap you'd initially planned. I hope it feels ok tomorrow.
It was blowing like a blowy thing this morning and as i lay in bed at 5:45 contemplating a 2 hour run, i remembered that i may be racing a 10 this Friday so off to my trusty old treadmill i went and sweated out 90 mins with 5-10 mins at MP to break the monotony of breakfast TV About half an hour later i nearly got blown off my bike on the way to work which nicely justified the treadmill run.
EDIT : I'm not sure how exactly the wind managed to nearly blow me off as i was drafting behind my 13 year old son at the time
If you're used to 20 a week i wouldn't push to more than 25 and off your 4 days would suggest something like
Tues 5 easy
Thurs 6-7 quality session including a good 2M w/u and at 0.5M c/d
Fri 3-4 recovery
Sun 10 easy
I'd say 12-14 long run is a bit much for your weekly mileage and not necessary for a 5 miler
If you can get 3-4 miles of quality in the Thurs session that should be sensible at around 12-16% of weekly mileage. General guideline is that quality mileage shouldn't be more than 15% of total mileage.