The OP states that the trip will be in June, so I don't think snow and cold temps will be much of a factor.
However, I do find that the cold does significantly impact my Mi/kWh. In the summer, I was able to easily bang out 4.5 mi/kwh on my commute to and from work. In the winter, I struggle to get above 3.5. Same car, same route, same driver. In the summer, I was only charging up to 80%. In the winter, I let the car charge to 100%. I found that in the summer, full regen would become available almost immediately after leaving my house, or as soon as the 10th bar disappeared. In the winter, because I'm leaving the house with 12 bars, and because it's cold outside, I'm finding that full regen is never available on my commute. Even at 6 bars (56% on my WattsLeft meter), only 4 of the 5 regen circles are lit up, and that's at 0 mph. At highway speeds, only 2 or 3 regen circles are available. Again, this is even at a relatively low SoC. This lack of available regen is, IMHO, causing me to get lower energy economy than in the summer. FWIW, my Leaf is a few days away from turning 14 months old, has just shy of 14,000 miles on it, is always parked outside (so we can assume battery temp is the same as the outside temp), and I started the car up this morning in 35F temps and my WattsLeft meter was reading 275 gids.
To the OP, I think you'll make it just fine. Slow and steady is the key to "long-range" driving in the Leaf. No jackrabbit starts, no sudden stops. Give yourself a lot of time to brake when coming up to a red light so that you get the full regen available. As others said, find a slow moving vehicle to follow on the highway. Maybe you'll want a copy of Tony's range chart with you -- I keep a laminated copy in my glovebox.
Former Ride: 2012 Leaf SL, Ocean Blue, Leased 1/27/2012, Returned 4/27/2015, 37.7k miles & 12/12 bars
Final LeafSpy readings: AHr 60.73, SOH 92%, Hx 90.90%, 250 Gids on full charge
Current Ride: 2012 Nissan Rogue SL AWD, Black