We had a dog on the farm that was scared of cows (actually he was pretty nervous about most things), if you were trying to round up sheep in a field with cows in he'd disappear and come up behind you and bite you on your backside! Let's just say he wasn't the only one that was nervous
In more recent times we had a Lab and the vet, who knew we were keen on fell walking and being outdoors, advised that we didn't take him on the fell until he was a year old to give his skeleton time to develop and firm up. He ended up being a brilliant fell dog and did lots of my BG training with me - cue comments of "It's a Labrador ... running!", he would even come out mountain biking with us and would run by the back wheel when on the road. When he was about ten we noticed him struggling, it turned out that his spleen needed removing so we took a lot of care of him after that as he had a habit of passing out!
Be patient and you'll have many years of fun.