Hi Danielle, and welcome to the forum! :-)
Yes, if you ride consistently, you will certainly become stronger and more able to tackle hills. One thing that beginning riders sometimes forget is to to learn to use your gears and then use them. I see so many struggling up hills in far too tough a gear. To go up a hill, you need to shift into progressively easier gears so that you can keep your cadence (pedaling speed) up fairly high. When you do that, you should be able to get up most slopes once you build up enough cardio conditioning to do so. With the low gearing on mountain bikes, your legs should be fine, but your heart/lungs will limit you until you gain conditioning. It's also no sin to walk when the going gets too tough!
As for the saddle, give it a little time. When you first start riding, or start back after a break, your sitbones are going to be sore and tender. Your soft tissue will become much more tolerant over time. If it didn't, not many people would stick with cycling! But the break-in period for a saddle can take a few weeks. If the saddle is really wrong for you and things don't improve, then you should consider a new saddle. For me, I need a saddle with a cutout and found that the Selle Italia Diva Gel Flo works the best for me. It has a large cutout and is medium-firm. Beware squishy, soft, wide saddles that seem like they would be super comfortable. They tend to be the worst over time for causing chafing and other issues.
Also very important is to invest in some good bike shorts that fit you well and have a good chammy liner with padding. I am partial to the Sugoi brand, but everyone has their preferences. If you feel uncomfortable wearing skin-tight bike shorts (as they should fit), you can wear them under a loose pair of shorts, but don't wear underwear or you will chafe your lady bits. They are made to be worn commando!
I hope these tips help. Keep us posted on how you are doing, and congratulations for taking charge of your health!
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow