Check out Rite in the rain notepads. I used to use them surveying and for field notes and they work great. They sell a special weatherproof pen, but I always just used a pencil with no problems.
Maybe try adding in another 50 or 100 meters each time, but as long as you're working hard, the amount of meters you do is less important. To vary things up swap in a kick set every now and again or maybe do some sprint 50's or 100's of your favorite stroke with 10 seconds rest. If you google "beginner swimming workouts" you should be able to find some sets to give you ideas. If you are wanting to make things harder, try making the intervals you're doing a bit quicker. For example if you're doing 4x50m on 2min intervals, try doing them on 1:50 next time, or do 6 of them instead of 4. You can experiment with different effort levels through a distance (or a set of distances), like doing a set of 4x100m as 75% effort, 85% effort, 95% effort and 100% full sprint. You could also do that as the 25m lengths of a 100m, or 200m or whatever distance you feel like doing that day. Play around with that and you should be able to keep the intensity up.
Definitely try to have fun with your swimming, if you're enjoying it you're less likely to quit on the hard days. I always found that variation was nice, but also having a planned workout for myself helped me to not give up early during the hard sets. Maybe have a journal? I dont swim that much anymore (although I swam for about 10years and later coached), but I have one of these waterproof notebooks which would probably work quite well for tracking your workouts. Then you could have it poolside with you.
Just getting in the pool and doing some meters on a regular basis is gonna be great for you though. The amount of meters you do isnt going to be as important as getting your heart rate up for an extended amount of time, as that is what is burning calories. If you're dead tired at the end of a workout, it doesnt really matter how many meters it took to get there. Also if you can budget time for a easy 50 or 100m of cool-down at the end of a workout, id recommend that. I'll do wonders to get the lactic acid out of your muscles and have you feeling better for the next swim.
Keep in mind tho that not every swim is going to feel good. Some days you'll feel like you can do a ton more meters, and some days it'll feel hard to do more than a few lengths. That's normal, so dont let it discourage you. Keep at it! Good luck and Welcome back to the pool!
PS. Dont forget to bring water bottle, and drink lots of water. You wont feel like you're sweaty in the pool but you can actually get pretty dehydrated.
Ha - the 'more 580 than eco' 698 is like an EF and holds a ton of ink. Dunno - for a trip like that I start to think durability is more important than ink capacity - a metal pen might weight a bit more but would not break in your pack and make a mess.
Suggestion: https://www.ebay.com/itm/DELIKE-Updated-fountain-pen-Metal-Brass-Exa-Fine-Nib-Pen-/262791689998
Also something I'd consider: https://www.amazon.com/Rite-Rain-All-Weather-Side-Spiral-393/dp/B001PD28JM
I dunno how they hold up to fountain pens - but there is something seriously badass about paper you can wring out and keep writing on.
I got one of these, so I could write the ideas down before I forget them.