SPAMming along...

[Spam cans and you]>....................100 miles.........................>

-you are standing next to a pile of 5-ounce Spam cans (pork/ham mix!)
-these are sufficient in number to complete this puzzle!
-you wish to walk a distance of 100 miles (straight path)
-to keep up your strength, you need to eat a Spam can after each mile you walk
-you can carry a limit of 60 cans, no more

So first, you need to place a certain number of cans at certain mile points;
as example, you could walk 10 miles, leave 40 cans at that point, then return:
you will have eaten 20 cans during this trip, right?
NO, you CANNOT do stuff like walk 3/4 mile then eat 3/4 of a can:
you MUST deal with full cans at ALL times!!

What is the minimum number of cans that can be at the starting point?

236 cans

1st trip
start with 60 cans, travel 8m, dump 44 cans, return

2nd trip
start with 60 cans, travel 8m, get 8 cans, travel 12m, dump 36 cans, turn around,
travel 12m, get 8 cans, return

3rd trip
start with 60 cans, travel 8m, get 8 cans, travel 12m, get 12 cans, travel 20m, dump 20 cans, turn around,
travel 20m, get 12 cans, travel 12m, get 8 cans, return

last trip
start with 56 cans, travel 8m, get 12 cans, travel 12m, get 12 cans, travel 20m, get 20 cans,
travel 60m (for a total of 100m)

number of cans consumed = 236

Correct. Can be shown this way also (m8 = mile#8...):

3 return trips and 1 one-way trip from start to m8 : 236 - 8*7 = 180 cans at m8

2 return trips and 1 one-way trip from m8 to m20 : 180 - 12*5 = 120 cans at m20

1 return trip and 1 one-way trip from m20 to m40 : 120 - 20*3 = 60 cans at m40

All over...60 cans with 60 miles to go!