Agility can be a boring and tedious skill. It can also be a quick and easy skill. It has no real purpose in the game, unless you want to wear an Agility cape if you have one.
Training Agility is easy. The main way of getting experience is to buy agility tickets from the pirate at the agility training arena in the wilderness. Because of this, you could just obtain a large quantity of tickets from someone and go purchase 99 agility in a few seconds.
To get to the agility training arena, just go to ::portals and enter the agility portal (it's right in front of you when you teleport). The fastest way to get tickets aside from buying them is to constantly teleport to (3002, 3945) and click the log, since it is the only obstacle that gives you tickets. This will save you a lot of time. When you have a good amount of tickets, trade them with the pirate to buy experience.