Mine are 3.5 years apart and I was dreading it too. I had the second around my 40th birthday too.
But I've been happily surprised at how ok it is. In fact, there are times I am thankful DC1 will entertain DC2 when I am getting showered and dressed in the morning, because I used to recall being the sole entertainer for an 'only' child, which was much more pressure.
The hardest part was during the day when DC1 wasn't at preschool and I was desperate to sleep when baby did during the day. When that happened I either enlisted a babysitter for an hour, or used babysitter-CBeebies
while I slumped on the sofa.
Still lovely to see them play together, to watch DC2 desperate to copy DC1, and I am being creative with how to deal with their fights over toys. I remember my uncle telling me that he had 2 boys, and didn't LET them not get on. He reckoned you nip it in the bud and they will accept it. So I went into having 2 kids wth that mindset, and I've often heard my uncle's voice in my head as I decide how to deal with the latest squabble!
The thing is, when you go out you take a baby coat and drink, so instead of 1 coat and 1 drink, you take 2! You have to strap one child in the car, so you just strap two in!
The great thing about that age difference is you only have 1 in nappies, your firstborn is likely to be able to at least partially dress him/her self, and use utensils at the table, etc, and they often want to be helpful big sibling.
I found reading a story about being a great big brother helped set the scene.
One thing to warn you though, don't be surprised if your firstborn regresses. Mine wanted to use a dummy again, but soon dropped it when he realised he could do what he wanted and we weren't fussed (outwardly: inwardly I was screaming "no! Put it down!"). But that passes very quickly.
I find the little stages really gruelling, but now, a few years on, it is sooooo much easier and so however hard it might be, it will only be temporary. They grow up so fast!