Today I did a workout that I learned from Brad Hudson, called 1-2-3-2-1-2-3-2-1. You start with a warm-up; mine today was a mile of easy running. Then you run one minute at one-mile race pace, jog for a minute, run two miles at 5K race pace, jog two minutes, run three miles at 10K race pace, jog three minutes, and so forth. Cool down with another mile or so of jogging.
I consider this session a great early-season workout, for a few reasons. The fact that the intervals are run by duration rather than distance gives it a slightly casual feeling (even though you're running hard!) that lends itself to the early season. There's no reason to hit the track for this one. Just do it on one of your regular road routes. It's a fairly challenging session, but not extreme, so you finish it feeling good about having done some productive work and suffered a little, but if you're really honest with yourself you have to admit it was a piece of cake compared to some of the sessions you will put yourself through later.
The blend of different intensity levels is also appropriate to the early season. One of your highest priorities in the base phase of training for races of any distance is to establish a solid foundation of well-rounded running fitness. You want some speed, you want some high-intensity fatigue resistance, and you want some endurance, and you want none of these things more than you want any of the others. Brad Hudson's 1-2-3-2-1-2-3-2-1 workout is a good one for developing well-rounded running fitness (although I can't say it does much for endurance).
If you've never done it before, it takes some getting used to. The tendency is to run that first one-minute interval too fast and pay for it later. You have to be honest about running the one-minute intervals at one-mile race pace, the two-minute intervals at 5K race pace, and the three-minute intervals at 10K race pace to avoid bonking before it's over. The structure of the session disguises the fact that you're running a total of 17 minutes at paces ranging between one-mile and 10K race pace, which is a significant amount of work.
One last tip: It is extremely handy to use a speed and distance device while doing this workout. It not only allows you to monitor and control your pace during the workout, but it also allows you to download it and analyze it in cool ways afterward. To start with, you can compare how much distance you covered in separate intervals of the same duration to see how well you paced yourself.