I've done an 8 to 9 hour sector with our now 16 month old 9 times in his short life, aged between 3 and 16 months, and 5 of those on my own with him (i.e. not also travelling with husband). Mostly in CX J, one sector in QF J, never with his own paid seat. However, always with the basinette position assigned.
Really, it hasn't been a problem for us. If you can choose flight times to dovetail with bub's normal sleep/nap times, and they sleep well usually when out and about (i.e. our son sleeps for up to 2 hours in his pram if we are out and about at his nap time), then a fair % of the time will be taken up with a sleeping bub who hopefully might still meet the basinette length and weight requirements.
The last 4 of these longhaul sectors, son has been crawling/cruising, though not yet walking - this probably helps. But truly, he is content to peruse books, eat, watch some IFE in small doses and sleep.