Ncueno-- are you doing any other sort of speed training currently? any particuklar reason you chose 400 meter repeats? Are you looking to improve your shorter race times (like 5 ks?) or working on your VO2Max?
You have an ultimate goal of running for a marathon, i know, if you arent doing other speed training, and not having a specific goal per say, i would suggest something different
i think you might REALLY benefit for a pyramid style track workout session (doesnt have to be on a track, obvioulsy)

to REALLY focus on condition, keep the "rest intervals the same distance, no matter how far the "work" intervals are.
do 1X200m, 1X400 m, 1x800 and 1x1000, then 800 then 400, etc, or work your way up to 1000. than go back to 200m and work your way up again. You could do stuff like 2X200, 2X800, 2X1600, 1X400, in other words, mixing it up. Either do a 1:1 ratio of rest/work (4oo rest for 400 work) or do a 2:1 ration (200 rest for 400 work) OR, like i suggeested previously, to REALLY amp up your conditioning, sdo the SAME rest for all the repeatrs..... maybe 400 meters rest in between for every distance, to start.
Here is my reasoning.. at this point, no running goal in the near future.. right? Doing 200 m and 400 meters really works on improving leg turnover rate, runing efficiency, V02Max, overall speed, etc. ANd doing 800-1600 repeats gives ou a chance to work on LActic Acuid threshold, which you knowmy opinioin on the benefits of THAT
My other reason for suggesting this style..... Now that you have a garmin, and are a more seasoned, and focused runner, i think its time, that you are reasy, to start learning and training your PACES. YOu no longer need to be a "same pace for every race" girl. Its time to start having 5K paces, and 10K paces and 1/2 mary paces. Its time to start figuring out what your threshold is, so you know what you have to work on, and youll have measureable, achievable results. Teaching your body to run certain paces, to be able to fall into them through "muscle memory" is a great great skill to have. you are going to do you 200 meters a ta very fast pace, (for you), you are going to try and keep ALL your 200 meter inttervals at the same pace during the workout. Yor 800 m to 1000m are gong tobe your LA Threshold pace... in al likeliehood it will be 30 seconds per mile slower than your 400 meter intervals, whoch should be SLIGHTLY faster than your 5 K pace, or so. It should be uncomfortable by the end if each interval, but you shouldnt be DYING.
Your 1600m are going to be at aLITTLE bit faster than your current, natural pace is... this is giong to help you improve that "comfy zone" pace. If you run 10 minute miles naturally, do the 1600s at a9 min mile, for example... its just ONE mile, and you should be able to run it that much faster without too much difficulty... its when you string 1 mile to10 MORE miles at that pace, thats when you cant run that fast, kwim?
Im sorry my thought process is SO SCATTERED and unorganized today. ASke me to clarify and give you specifics on anything i just said... i can also help give you some good paces to start with on the distances
oh ya, andf changing up the interval distancee also keeps things from getting old
Hope i made at least SOME sense??????
BTW, i NEVER re-read or proof read a post befor4e i hitreply... EVER lol