In point of fact, this isn't the full story. You can definitely preserve a lot of your fitness in the pool, but the brutal truth is that you will be working harder and longer for equal or slightly less benefit.
I have generally stuck with a traditional training schedule -- a weekly interval workout of 800m repeats (each would normally take me about 3 minutes), a weekly tempo run of 20+ minutes, and a weekly long run). But I have found it necessary to modify it slightly to keep the same level of effort. These changes are listed below.
- Recovery: The pressure of the water assists your heart in pumping blood, and the water itself cools you down rapidly. As a result, you recover much quicker in the pool than you would on land. On land, if I were doing 800m repeats in 3 minutes each, I would take about 2:30 recovery -- this would give me enough time to catch my breath and regroup, but I would NOT be fully recovered before the next interval (in VO2Max training, that's the point -- you want to be a bit more fatigued as you start each interval).
But in the water, 2:30 rest gives me full recovery. It's way too much. In my experience, about 1:00 recovery in the water is equivalent to 2:30 on land, and so that's what I use (for other recovery durations, I just use a formula of 2/5ths of my land recovery duration for the water).
- Perceived Effort: I'm not sure quite why this is, but perceived effort is not accurate in the pool. For some reason, what should be an equivalent level of effort seems to result in much more suffering in the pool (this has been my experience, and also the experience of many other successful pool runners that I have discussed this with). 10K effort feels like 5K effort, etc.
Thus, in order to get the equivalent workout, you have to brace yourself to hurt MORE in the pool than you would on land. 3:00 minute repeats really should be quite painful (much more than they would be on the track), and shorter intervals near-excruciating. Yes. It sucks. It really sucks, sometimes. But one plus is that this develops mental strength incredibly effectively, which should convey well to land running.
- Duration: You'll try and try to correct for the difference in perceived effort in the pool, but the truth is, you'll never quite get it. So, you have to address the issue by also extending the duration of your workout. For me, I've found that the best rule of thumb is to nearly double the length of the workout. A track workout of 6 800m repeats on land becomes 10 intervals of 3 minutes each in the water; a tempo of 20-25 minutes on land is 40 minutes in the pool.
How do I know that this is the best conversion? Well, I honestly don't. But I can observe that the fatigue I experience in the afternoon after a morning pool workout of 10 repeats is very close to what I would feel after 6 intervals on land (ditto for tempo duration), and that's enough for me.
On a similar note, I've noted that I do 3+ hour long runs in the pool, but I don't feel anywhere near the same post-workout fatigue from a "long run" that I would feel after a long run lasting 2-2:30 hours outside. Again, you have to go longer to get the same results.
[and no, I don't do 4:30 hour long runs in the pool -- I don't have that much time. At some point, real (non-running) life does control.]
And this (among many other reasons) is why land-running is much more fun than pool-running. But the pool-running can work. You just need to work to make it work.