I want to pick up my kendo but I'm afraid I'm a bit out of shape.
Does anyone know a good training scheme to get into shape and be ready by September (5 - 6 months)?
Without knowing what your goals are for getting into shape, or how you intend to get back into kendo, all I can do is comment on how I approached getting back into karate. I had a lengthy layoff from karate (about 2 years) due to issues with psoriasis on the soles of my feet. What I didn't know at the time is that the intensity of the psoriasis was related to my body weight. Long story short, I got to my "sick of being like this" state and lost a lot of weight. The psoriasis cleared up, but I lost a lot of strength in the process. So I attacked getting back into karate in a couple phases:
General Strength and Conditioning
To redeem my strength, I started a simple barbell training program. Now, that only addresses one aspect of what I lost. But I did find going up in weights on a regular schedule quite addicting.
Next up was conditioning. The problem of being able to keep my breath during the rigors of martial arts training is no small task. Barbells don't normally do that (there is a way, but off topic for this reply). I employed a couple of approaches:
The key to conditioning for martial arts is to make the effort you spend similar to what you need for both classes and taikai. You have periods of rest and activity. If a round in a match is always 2 minutes, and you have no more than three rounds (TKD tourneys are like that), then you need to at least be able to do that.
For the most part, my knowledge was largely in-tact. However, the different combinations and katas were quite rusty. I did find resources to help jog my memory of the sequence of my katas. Some of the katas were very simple, and I remembered without help.
However, I didn't wait too long before asking my sensei if I could rejoin class. We worked on getting back to a certain level of knowledge, and are still working on it. There is a lot to cover.
Where I am now
I'm back. I'm in better shape than I ever have been, even though I'm not a young guy anymore. I've figured out how to make the general strength and conditioning support my martial arts training, partly through trial and error, and partly through learning from others. There's still some gaps in my knowledge because I haven't used it for so long, but those gaps are being filled.
Why prepare at all
This is marginally true, but misleading. Runners and baseball players lift and work on conditioning, too. Judoka and karateka and kendoka even have a name for it: hojo undo, supplemental exercises. These things are important: being weak, slow, out of shape, and unable to perform basic movements will hinder your martial arts progress.
Getting better at any practice involves lots of doing that specific thing, but it also often involves developing basic physicality or maximizing one's physical attributes. To discard supplemental or preparatory training altogether is folly.
Training time in martial arts is precious. Class time, whether group or one-on-one, should be spent on activity-specific technique, drilling, and sparring. Using that time for general strength, mobility, and conditioning wastes everyone's time. Sport-specific strength and conditioning is of course necessary, but instructors must assume a basic level of physical competence.
What to do
Any general strength and conditioning program should be sufficient, unless you have a specific hindrance. In most cases this involves some degree of barbell or other resistance training, as well as running, with a possible emphasis on sprinting rather than marathon distances.
With resistance training, be sure to emphasize athletic work rather than bodybuilding. That is, whole-body movements, using compound exercises like squats, deadlifts, presses, and Olympic lifts. Chin-ups are excellent as well. The goal is not performance in any of these specific activities, but rather overall strength and mobility. Rippetoe and Kilgore's Starting Strength is a good reference manual, though the specific program may not be appropriate for you while doing other training.
You'll be able to dabble in kendo no matter how you prepare. But you'll do better the more physically prepared you are.