Training for the 400/800 is very difficult as it is the crossover point whereby aerobic energy pathways begin to take prominence over anaerobic pathways. In my opinion, a 400/800 runner needs to have the speed of a 200/400m runner and just enough endurance to maintain speed over the 2 lap event. The 4/8 runner's mile time will not be especially impressive compared to the shorter events (it's widely regarded Rudisha would struggle to break 4 minutes for the mile). Training for the most part is designed around treating the 800m as a very long sprint (therefore a low number of high quality repeats).
Not much slow mileage over the winter. Most 4/8 runners won't run more than about 60-70 minutes as a long run (and some top out at 45 minutes to get just enough endurance), supplemented by 2 or 3 25-40 minute runs. The base endurance stuff for this type of distance is basically your VO2 max sessions (so nothing especially 'slow'). Keeping in mind the need for sprinter-like training, sessions like these:
3*1M @ 5kp
3*1k @ 3kp, both with longer (2+ min) recoveries
15*200 @ 75% of 200m pb (45s standing recovery)
will improve endurance and strength in a different way to the tempos that your 800/1500 type runner would do to improve endurance and aerobic strength over winter. Remember we're not looking to run a spectacular 5k, 3k or even mile here (although improving the mile pb is certainly useful), we're looking to improve endurance enough so that the runner doesn't fade on lap 2, but not impede speed by running lots of mileage.
Fartleks are also very useful, running lots of very short, fast repeats (akin to the 15*200 mentioned above) to improve speed & economy but also giving an aerobic boost.
Improving basic speed over winter is a relatively simple task case of 5 or 6*10s hillsprints w/3 or 4 min recovery. You can work on your starts too (very short, maximal hillsprints, really focusing on driving and turnover) if you want to help with the 400m aspect, but it depends on if you think you are more likely to be competitive over the 400 or 800. I would also recommend doing plenty of strides both before and after sessions.
Spring is the transition into the harder race pace efforts that are needed to race the 400 & 800m well. I would recommend focusing on mile pace with longer recoveries primarily. Focusing on 400s & 600s as mile pace and 150s & 200s at 'comehome' 400m pace (the pace you expect to run for the second 200m of a 400m race) provided harder aerobic work but also a transition into anaerobic processes as well. For example:
5*600 (3'00 rec) @ mile pace
5*200 (2'30 rec) @ comehome pace
3*1200 (3'00 rec) @ 2-3s/lap slower than mile pace
are examples of strength-based sessions you could run prior to sharpening your race fitness. Remember there is still a sprinter's bias here of quality over quantity.
When it comes to getting race fit, really focus on getting the maximum out of each individual rep. Sessions to run are ones like:
3*600 @ 90% of 800m pace w/10'00 rec
3*300 @ 90% 400m pace w/10'00 rec
2*400 @ 95% 400m pace w/12'00 rec
3*800 @ 95% 800m pace w/15'00 rec
3*500 @ 98% w/20'00 rec
2*250 @ 100% w/25'00 rec
The idea is not to run 10*200 or 10*400 as an (endurance-based) 800/1500 runner would do.... this is sprinter's training, every rep is very hard but the recovery is full. The aim is for very high levels of lactate on each rep, and to get rid of it all before starting the next rep. Regular time trials are also recommended (to gauge strength, 1200m TT's are excellent, for speed, 150/200m ones are).
Training for 400/800 is very difficult because you need to take a sprint philosophy and - to a certain extent - apply it to a distance race. The key balance is maintaining speed and strength while building just enough endurance to last for 2 laps. A 400/800 runner will typically run the first lap much quicker than an 800/1500 runner with the same 800m pb, due to their superior speed. The difference being the 800/1500 runner will slow down less. It is possible to make it work, but the balance must be there.