Runner Clive Fitzpatrick, 44, was driving on the A451 back home to Stalybridge, Greater Manchester, after a competition in Denbighshire on 15 July when the accident happened.
He went on the wrong side of a fork in the road and hit an oncoming car.
Fitzpatrick was convicted of dangerous driving at Mold Crown Court and banned from driving for 12 months.
Judge Rhys Rowlands told him he could not imagine anyone "in a million years" failing to realise they were entering a dual carriageway.
He said Fitzpatrick, who was travelling with his wife and baby in his BMW, had been guilty of "a woeful lack of attention."
Student Jessica Broom, from Wrexham, was driving along the road at nearly 50mph (80km/h) in her Renault Clio at about 21:40 BST when she saw Fitzpatrick's car coming towards her.
The two cars crashed and Ms Broom needed hospital treatment for her injuries.
The court heard Fitzpatrick's injuries stopped him from becoming a member of the British fell running team.
He claimed the sign in the middle of the road at Pontblyddyn in Flintshire, indicating which side to drive on, had been obscured and genuinely thought he was on a single carriageway.
Fitzpatrick was also ordered to take an extended driving test, pay Â£250 in costs and must carry out 100 hours of unpaid work.
A man who caused a crash after driving the wrong way on a dual carriageway has been given a 12-month community order.