Horkers are great to use at leveling skills due to how slow they are.
Get an unimproved dragon bone bow (or use bound bow). Fire quick, weak shots. Damage doesn't matter, only the base quality of the bow.
600 shots is probably not nearly enough to reach mastery. I haven't counted because I always have more iron arrows than I know what to do with.
Just be glad it's not destruction where it takes 500 shots to go from level 99 to level 100.
Practice, train, practice, practice, train, practice.......
Wash, rinse, repeat Sir.
That should get you you into the low 80's in no time.
Help Faendal. Let him teach you, take his gold back. If your archery is too high, do the same to Aela if you can get her as a follower. Otherwise, it's just another combat skill, it'll go up decently the more you remember to use it in combat.
Is http://uesp.net/wiki/Skyrim:Angi one of the NPCs you "used up?" If not, doing her quest can get you another six levels of Archery for free, but it's a one-time only sort of deal, IIRC. She's south of Falkreath, on one of those winding roads up the mountainside. You'll know you're on the right path when you start seeing bear traps.
What standing stone are you using ? i recommend the Lover stone. Use Faendal as a cheat if your wish (i didn't), help him in his quest, get together 1000 gold & he will train you & you can take the money back, rinse repeat upto 50. When your in the 80's or 90's level in archery, visit Angi's camp for free training.
It levels pretty easily for my previous bowmen, who was close to 90 in archery by level 32.
Start every fight you can with your bow......and hopefully finish it with a bow.The SKYRE mod(if you start a new game) has an extensive bow and arrow perk tree and is my favorite mod .
I would also give serious consideration to NOT focusing on advancing the skill. Just do whats fun, and let the game entertain you...however that happns.