Final Training Push During the Season
Although I'd always recommend staying in good physical shape, due to different circumstances, we all tend to lose some fitness during the course of the season. Sometimes matches are short, injuries hamper your schedule, lack of motivation can strike and also there is the problem of just getting too busy. Whatever the reason, or even if you feel strong physically, now is a good time to put some time in to boost your fitness levels.
I don't think going back to a summer-type training schedule is smart or worthwhile as you're more than likely playing lots of squash during this period and want to keep your form. Lots of shorter, more game specific training, is ideal to ensure you stay comfortable with your movement on court. If the season so far has been tough, keep the physical sessions even shorter to ensure you don't get completely fatigued.
What I'd suggest is adding manageable blocks of physical work onto the end of current squash training, practice or matches. These can include ghosting, on court sprinting and circuits—all that fun stuff! The beauty though is that it can be a 4/5-minute session, which, after practicing or playing, is a nice little top up.
Ghosting: stick to short and sharp periods of work. I'd either count 10 shots or work for 20 seconds. Take little breaks and go again. If you are working with a partner, alternate continuously for the allotted time. Move quickly and go for full exertion; don't leave anything out there. Around 10 sets would be a good amount and is only 3 minutes 20 seconds work.
On Court Sprinting: simple sprints up and down the court are an easy way to work hard physically while still remaining pertinent to squash. I'd look at running 20 lengths as quick as you can, take the equal amount of time resting and then repeat 3-5 times. Make sure when turning at either end of the court, land like you would for a shot, keeping it relevant to playing.
Circuits: an excellent way to replicate the pattern of your heart rate in a match. These can be done on court with squash movement patterns as some of the stations alongside the staples—burpees, push-ups, sit-ups, etc. Check out the ever-growing list of circuits on the Fitness Wall of the website (SquashSkills.com).
I used these to keep my physical form during the season when longer training periods were not available. Try it and see if it makes a difference to your game. It could change that 5 set loss into a win!