The quest for enough reputation for another race's mount is very long.

This is a link to a mod that lets you see what your reputation numbers are:
Reputation Tracker

Here is a list of reputation per level of faction:

Hated: 36,000 reputation
Hostile: 3,000 reputation
Unfriendly: 3,000 reputation
Neutral: 3,000 reputation
Friendly: 6,000 reputation
Honored: 12,000 reputation
Revered: 21,000 reputation
Exalted: 1,000 reputation

As a non-night elf, you start out at 100/6,000 Friendly reputation with Darnassus. In order to purchase training to ride a mount, you must be Exalted with Darnassus. You need to gain 38,900 reptutation.

When you do the 20 runecloth quest "Additional Runecloth", you receive 50 reputation per turnin at level 50. That means you need at most 778 stacks of runecloth (less whatever reputation you have gained from quests).

You need to do the Donation of Wool (60 wool turnin at level 15), Donation of Silk (60 silk turnin at level 25), Donation of Mageweave (60 mageweave turnin at level 40) and Donation of Runecloth (60 runecloth turnin at level 50) before you can get the repeatable quest Additional Runecloth (20 runecloth turnin) at level 50.

As of patch 1.9, the "Additional Runecloth" quest will have its level increased, though they are not raising the minimum level to start the quest. What this means is that level 60 characters will be able to do the "Additional Runecloth" quest and receive full reputation gains. Previously, level 60 characters were not getting the full 50 reputation for stacks of runecloth. So, it will be easier to raise reputation with allied races at level 60 after patch 1.9.

You gain full reputation by completing green or higher quests. Once a quest is grayed out, you will receive less reputuation. The more levels the quest has been gray, the less reputation you will receive.

If you want to buy an epic mount, find someone with Honored reputation (10% cost reduction) and Sergeant PvP (10% additional cost reduction) for a total of 800 gold for the epic mount instead of the mere 900 gold that having Honored will give you. 100 gold is 100 gold. : )

Edited, Sat Jan 21 02:42:43 2006

Also note that you cannot receive this quest until you have completed the first 3 he has to offer. A Donation of Wool, A Donation of Silk, and A Donation of Mageweave. After completing these 3, the runecloth quest will become available.

Each of the races has a seperate NPC that gives this quest. The NPC is usually located near the tailoring trainers in the race's capital. Turning in the quest to that race's NPC also gives you a significant amount of reputation for that race.

After the initial hand-in, this quest becomes repeatable. For every stack you hand in (as opposed to the three stacks initially), you get the same amount of reputation per runecloth turned in as the original quest.