It's perfectly understandable to be apprehensive about LEL. It's 1400km and covers some challenging terrain. Add in the vagaries of the British weather and the fact that you pass through the Wacky Races county of Lincolnshire twice - the second time when you're very tired - and you have a ride that for the average weekend cyclist would be a stretch too far by a considerable margin.
But, you've already demonstrated that you're not the average weekend cyclist. Even if your 300 was a struggle, you've more miles under your belt from one day than the average rider achieves in a week. Between now and the end of July, I'm sure you'll get at least one 400 and one 600 done, together with lots of other riding. And over that time, your fitness will improve and, most importantly, if you do enough Audax events, you'll get even better at managing your time off the bike and your food over extended periods.
Come the end of July, you'll certainly be ready to start LEL and, so long as you don't beat yourself up over the time, have a great ride. If turns out that you're not making the progress that you would have liked, the 'out and back' route means that it's easy to call it a day at almost any point and enjoy the ride back, still using the controls, and have had three or four good days out. Of course, you won't do that.
I know that feeling of trepidation. Two years ago at the end of my qualifying 600 for PBP I was having serious doubts about taking up my place. I'd had a s**t time on the ride and couldn't envisage riding twice that distance, even though I had two PBPs under my belt already. Thankfully, LWaB and H&K from this forum laid into me and persuaded me that I was simply feeling the inevitable doubt that comes after a difficult ride and I went on to ride a comfortable PBP '15.
Similarly, as I have mentioned elsewhere, I've also experienced a disastrous 300 and a challenging 400 in the past month, but I'm taking the view that, as with PBP, I've done it before so I can do it again. Or at least I can start it again and see how it goes. No matter what, I'll be riding my bike and what's not to like about that? So my advice is: go for it and enjoy the ride,no matter what.
See you in Loughton