As everyone else has said it will get better - they don't call it marginal gains for nothing!
You have a target in June to work towards which is great, personally I find it easier to breakdown the time inbetween and set targets for where I want to be each month, i.e. 20 mile ride by the end of September, 30 mile ride by the end of October, etc or a total mileage for the month, 25, 50, 75, etc. towards your goal of June. You should have a rough idea of daily mileage required for the ride (I can't imagine it exceeds 100 miles a day), aim to get up to 75% in one ride of whatever distance they expect of you (or you expect of yourself) by the end of May next year - adrenalin will get you through the extra 25%!
Don't worry about speed at all at the moment, that will come with time (but no harm keeping a note
) - just get your positioning right and enjoy your time on the bike, as I said being goal-orientated works for me but it's not for everyone. In time you'll be amazed you ever went so slow or bothered going out for such a short ride!
Good luck and keep posting! Regards, Mark