If you look around training programs, you'll see that there are two camps for coaches: those that give you training programs by distance, and those that train you by time.
If you don't know the difference, it simply means that some coaches and/or training programs tell you to train by swimming/cycling/running a certain distance per workout. The theory is, that if you hit a certain amount of distance at the right time and amount, you'll be able to complete a race. Training by time, in contrast, focuses on being out there and swimming/cycling/running for a certain amount of time and not be concerned about the exact distance covered.
In moving from beginner to intermediate triathlete, I have found some interesting elements to training with either:
1. I started training for my first marathon by training with distance. As a beginner, I think that training by distance is more advantageous to you than training by time. This is because you don't know exactly how long it will take you to complete a race. WIthout knowing, you may be under-prepared for a race if you trained using a time system geared towards more experienced athletes because more experienced athletes will be able to cover more distance in the same amount of time. By actually going the distance, you can train your body and mind to know how long it will take to complete a race, to prepare it for surviving the distance both mentally and physically, and to know what will happen to your body, joints, and mind for being out there that long. Otherwise, you'll find out at the race itself and it could be rude shock to your system, which I feel is totally unnecessary if you had prepared sufficiently.
2. Once your experience with racing grows, you get faster, and you start knowing your body better. You can then start moving towards training with time, knowing when you can approximately complete a race. You can just train with quality up to about that time and you'll be prepared by race time. An example would be when I was training for Ironman Brazil and I would do these 30+ minute hill repeats up Old La Honda, which was only 3.3 miles to the top. I would do enough hill repeats to be out there for 5 hours, but my total distance would only be about 50 miles. But yet this gave me enough fitness and strength to complete the much flatter Ironman Brazil bike course of 112 miles in 6:15.
3. With swimming, I use a combination of distance and time. I swim one 45 min, one 1:30, and one 2 hour workout each week to prepare for Ironman. Each workout varies in distance depending on the workout itself. But I know that in my 2 hour workout, I will cross 4000+ meters which will be sufficient enough to complete the Ironman swim.
4. When training for triathlon, the cross training factor really helps. All the mileage I put in on the bike really translates to fitness on the run. Thus, when I peak for Ironman, I usually am swimming a 4000+ workout, riding a 6 hour ride and do a run of 2:45 in the same week. All that saves me from killing my joints and needing to run 3+ hours, which some training programs tell you to do. In that 2:45, I usually reach a max of 16-17 miles; hardly the 21-24 miles that some training programs tell you to do. But I know that the cross training effects mean raised overall fitness and I don't have to go the actual distance.
5. Knowing your race times means you can train for that time, and not worry so much about distance. For example, I know that I can ride the 112 miles of Ironman in 6-6:30 hours. Thus, if I ride with quality (not just cruising) for about 6 hours, I know I can complete the 112 miles of a race without actually covering 112 miles in training.
6. By the way, you can't be cruising in training with time. You need to train with quality and smarts and not just think that if you jog or cruise the time, you're going to be OK when race time hits. That happened to me during Ironman Austria. Thinking that I could just ride the same rolling hills course to prepare really screwed me when I got to the hills of Austria.
Switching from training with distance to training with time was a real signal to me that I was getting truly familiar with my body and fitness level, and knowing what it takes to train for Ironman without overdoing it and risking injury.