Hi,
Sounds like you’re doing a lot!
Combining marathon training, HIIT and regular heavy weight training after extended time off sounds way too much. I guess you have lots of options depending on what’s the most important to you.
If you’re really enjoying the heavier weight training I’d stick with it and focus. If it’s an option, I’d personally not do the marathon.
For HIIT, one session, maximum 2 sessions per week, is enough. Perhaps even too much depending on weightlifting volume. Also depends on what you mean by HIIT.
For aerobic, I’d take it really easy and just build a base. If weight training, brisk walking, slow jogging or moderate cycling will help save the legs for weight training while building the aerobic engine.
For weights, if possible, I’d reduce the intensity of each session and do more sessions rather than fewer hard sessions that knacker you out.
Or, reduce the heavy weights (go lighter for fewer sets) and focus on the marathon training.