Your aerobic endurance stuff can be almost anything really. As long as you keep it your body in around zone 4 you can swim, jog, bike, whatever. Back when I boxed in hs i would use skills training as a form of aerobic cardio. One thing I brought over from my experience in Shorin-ryu karate was using katas. I remember having to perform lots of these routines every time when I was being promoted a belt. Using this concept I would take sequences from the boxing gym and turn them into a kind of shadowboxing kata if you will. One of my best combinations was a double jab, right cross, slip, liver shot, slip, movement. So on the days i was doing my steady state runs, I would finish my run with 30-45 minutes of drilling combinations and sequences I learned in the gym and from sparring. For example, if i was in the gym and I threw a really good combo or sequence in sparring, I would remember exactly what it was and drill it during those post run shadowboxing sessions. These shadowboxing katas I did at a moderate pace keeping my heart rate at around zone 4 and focusing on absolutely perfect form. You have to make sure that if you're going to do this kind of drilling that your form is perfect because otherwise it will instill bad motor patterns. These sessions not only helped my endurance, but also were a way of getting some skills training in too. Just some other Ideas.
Now, I don't box anymore but I can shed some light on endurance training as I am a competitive cyclist and triathlete. Swimming is awesome as it is low impact, and will condition your upper body which is helpful for boxing. However, if you don't have prior swim experience, don't bother. Technique is SO important in swimming and if you start logging yardage with bad technique, you will screw up your shoulders guaranteed. And screwed up shoulders is really bad news for a boxer. You can do technique drills to help improve your stroke, but you're not training to be a swimmer, you're training to be a boxer so you could be using your time elsewhere. So as far as swimming goes, do it if you have semi decent stroke form and you can work on it a bit as you train, but if you're new to swimming, or don't have much experience, id skip it. This is not to turn you off swimming of course, I love swimming it is so peaceful and relaxing and trains your whole body. Not to mention it is a really valuable life skill. But in your case, you're simply trying to be the best boxer you can be, so spending a bunch of time honing your stroke is kinda pointless right now, but maybe something to look into down the road. If you do end up swimming I'd highly encourage you to warm-up and cool down before and after every session with lots of shoulder mobility work and make sure you do some rotator cuff stuff as well to prevent swimmers shoulder.
I'd still like to recommend cycling as, from my own experiences and the information i've seen, its the best way to improve your cardio. There is a reason that the list of highest ever recorded Vo2 Max numbers of all time is dominated by pro cyclists. Just nothing compares to 15% grade hill climbs, 120% V02 max time trial sessions etc. in terms of building iron lungs. However, cycling is SUPER expensive so beware of getting hooked on a costly hobby. But some slow zone 4 mileage on a cheap mountain bike is good too, and you don't NEED to spend a ton of money on gear.
The last day is simply supposed to be anaerobic conditioning, so hill sprints, track work, sledgehammer swings, tabata intervals all that stuff could be used. Hill sprints however are a staple in most fighters' training plans for a reason as they are brutal and effective. Track work is awesome too. I posted some track workouts in a cardio thread we had in the S&C section here, so if want to dig those up there is some really good workouts. However, right now in your training, I dont think you need super complex track workouts. I'd say 400m repeats are perfect for your needs. If you really want to mix it up do some pyramid or ladder training like this
rest same as 200m interval duration
rest same as 400m interval duration
rest same as 600m interval duration
rest same as 400m interval duration
these are just some ideas if your training started getting stale. Lots of options for progression too by playing with your rest intervals and intensities and interval duration. Honestly though, something really simple though like 10x400m with 400m walking active recovery sounds like what you need. I do have a whole training log book full of all kinds of nasty run, swim and bike workouts so if you do want some I'm happy to oblige.
Hope this helps, happy training!