Unfortunately, there are no shortcuts to this it really is a case of getting out there and running as much as possible. It's also not enough to just pound the streets; you need to have a good balance in your training of solid, steady running, mixed with faster, more up-tempo bursts.
These up-tempo runs consist of interval training, which means you differ your speed within a run. You could start off doing eight three-minute runs at a very fast pace, with one minute's brisk walk in between each.
This minute of walking gives you a chance to recover, but keeps your heart rate from dropping too much. This is important because you must stay active during this recovery period.
Your slow, steady period involves running for longer at a steadier pace. That may mean anything from 20 minutes for novices to a three-hour run for more experienced athletes.
If you are specifically training for the marathon, I suggest running four to five times a week to build up the stamina you'll need. This should consist of both steady runs, three times weekly, and up-tempo runs twice weekly.
Going to the gym to do some weight training also builds stamina, and it's a crucial part of your training. You can't expect to become a top-class runner without putting in a couple of sessions of weight training each week.
Your goal should not be to bulk up like a bodybuilder, just to tone up and become leaner and stronger. As well as leg exercises, such as lunges and squats, it's important to work on your upper- body strength arms, shoulders, back and abdominals.
Only when your whole body is strong can you become a better and more efficient runner.
For most runners, weight training twice a week is sufficient. Speak to the trainers at your gym about which machines, exercises, weights and repetitions will best help you meet your specific goals. They will also be able to show you proper lifting techniques, so you don't injure yourself.
Mental stamina is a different thing altogether. Running is quite tough mentally, and a running partner can really help you through the bad patches.
Everyone has their own mental ability. While the strong-minded will cope with the boredom better, others may need more support.
It can sometimes be hard to get motivated, but if you've set realistic, achievable short-, medium- and long-range goals, that should help.
Remember, everyone has training plateaus, when stamina and ability seem to level out, and your program is no longer a challenge. When this happens it's time to intensify your training slightly, set new goals and break through to the next level.
Liz McColgan MBE, 38, is one of Scotland's most successful long- distance runners. She won Olympic silver and a Commonwealth and World Championship gold for the 10,000 metres. She has won the London, Tokyo and New York Marathons, and now runs health clubs in Carnoustie, Perth and Methil with her husband, Peter. They have four children.