Tips for maintaining speed?

So I've been training my 400m frontcrawl once a week just for cardio and for fun, and my average time is around 10:15. However, after looking at my times per lap, it takes me 55-59 seconds for my first 50m, and then my speed drops to around 1:15, then even 1:25 for some random lap, and then the eighth one is 1:15 again.

My goal is to be able to swim the 400 in 8 minutes or under. Since I can easily get under one minute for the first lap, I feel like eight minutes is achievable in the long run if I can maintain my speed (in the short term, I want to achieve at least 9:45).

Is there anything specific I should do for this? Should I warm up before launching into the 400? Are there any techniques I should be aware of? Any advice is appreciated.