Extra low cost
The reference sheet I use:
Example
If RT11 has 3000 finished heroes and RT20 has 150 unfinished heroes ( waiting to train or 300 days ) then you have enough heroes and food for 2x 5* 4.80.
Pace
I have done this at least 15 times on my various accounts. I keep promising to remember to pace myself.
Set a timer and stretch every 15 minutes.
Drink plenty of fluids.
Buy a stylus.
When I forget, my hands are usually sore for several days.
Order
I tried this a few times. You can also try and save all your same color 2* heroes:
But.
I have 50 open roster slots. It takes 1,259 from Extra low cost training to go from 5* 1.1 to 5* 4.80 . So I am doing this at least 25 times.
Since you are doing so many times, the food savings gets lost.
Saving trainers heroes is nice, but those are more slots I lose. If I save ten trainers, that leaves me with 40 open slots which means 30 times collecting from camps instead of 25 times. Which means clicking on Layla 20% more and Sigrunn 20% more and Ragnhild 20% more.
My new methods:
Sort by power
Use top 10 highest power heroes to level
repeat until empty
get more heroes
Because you sort by power, the new heroes always go to the bottom.
Because you are using all the heroes, you do not need to waste time scrolling to the bottom, paying attention to colors, etc.
Trainers and tokens
Edit:
The problem with saving tokens for future Hero XP is claiming them takes forever since you must do a server call each time. Never again.
I saved 151 silver Daily summons tokens for leveling heroes from Sand Empire/ Ranvir ( no Ranvir 
2x Hisan 
) and it took forever. Though Hisan did enjoy the Carver and Brienne I summoned.
Now I use silver Daily summons tokens and Trainer heroes when I get them.
And save heroes in Extra low cost training instead.
Food Budget
Food required
Hoarding food for Extra fast
Yearly food needed to use Extra fast training ( pre Atlantis Rising so Advanced Farm might be a good idea )