I hesitate to stick my nose into anything Charlie offers his advice on - and I couldn’t comment on the issues of training indoors - but in case you were looking for some general ideas and discussion on training for the 400m the following link will take you to a summary version of War & Peace, err, the (dreaded) “lactate threshhold” thread which deals exclusively with one lap wonders.
hmm, just tried it and hasn’t linked up for me (Pindaman help!!)
Strange, the link posted by Pindaman on page 113 of the threshold thread works perfectly. My IT IQ is about zero, so if you’re interested, go to p113 of the “lactate threshold” thread in the Fundamentals section.