The 65-year-old star and 39-year-old Penny announced in August they are expecting their second child together but the 'Maggie May' singer insists the IVF (in vitro fertilisation) attempts took their toll on the couple.
Speaking on UK breakfast show 'Daybreak', Rod - who has a four-year-old son Alistair with the model - said: "It took us quite a long time - two and half years I think it was - it was an emotional rollercoaster."
Rod - who has six other children from his previous relationships - is adamant the new baby will be his last.
He added: "We're not trying for the football team, The Office is closed now, this is the last one!"
The musician has to work hard at keeping in touch with all his older kids, but has now found the perfect way to do so.
He said: "I love emailing all my kids, I'm good on emailing, but I don't even have a mobile phone."
Rod previously revealed he and Penny had to have IVF three times before conceiving.
He admitted: "It was a tough road. She had to inject herself every day and the first two treatments in the US didn't work. I couldn't bear seeing the sadness on her face. You have to wait for weeks to know whether it's worked or not, it's crushing when it doesn't."